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Some recent papers

11 January, 2020 in expository, math.AP, math.CA, math.CO, math.NT | Tags: Chowla conjecture, finite time blowup, Frank Merle, Hong Wang, Igor Rodnianski, Jeremie Szeftel, Larry Guth, Mark Shusterman, nonlinear Schrodinger equation, Pierre Raphael, restriction theorems, Ruixiang Zhang, Sarah Peluse, Szemeredi's theorem, twin primes, Will Sawin | by Terence Tao | 4 comments

Just a brief post to record some notable papers in my fields of interest that appeared on the arXiv recently.

“A sharp square function estimate for the cone in ${\bf R}^3$ “, by Larry Guth, Hong Wang, and Ruixiang Zhang. This paper establishes an optimal (up to epsilon losses) square function estimate for the three-dimensional light cone that was essentially conjectured by Mockenhaupt, Seeger, and Sogge, which has a number of other consequences including Sogge’s local smoothing conjecture for the wave equation in two spatial dimensions, which in turn implies the (already known) Bochner-Riesz, restriction, and Kakeya conjectures in two dimensions. Interestingly, modern techniques such as polynomial partitioning and decoupling estimates are not used in this argument; instead, the authors mostly rely on an induction on scales argument and Kakeya type estimates. Many previous authors (including myself) were able to get weaker estimates of this type by an induction on scales method, but there were always significant inefficiencies in doing so; in particular knowing the sharp square function estimate at smaller scales did not imply the sharp square function estimate at the given larger scale. The authors here get around this issue by finding an even stronger estimate that implies the square function estimate, but behaves significantly better with respect to induction on scales.
“On the Chowla and twin primes conjectures over ${\mathbb F}_q[T]$ “, by Will Sawin and Mark Shusterman. This paper resolves a number of well known open conjectures in analytic number theory, such as the Chowla conjecture and the twin prime conjecture (in the strong form conjectured by Hardy and Littlewood), in the case of function fields where the field is a prime power $q=p^j$ which is fixed (in contrast to a number of existing results in the “large $q$ ” limit) but has a large exponent $j$ . The techniques here are orthogonal to those used in recent progress towards the Chowla conjecture over the integers (e.g., in this previous paper of mine); the starting point is an algebraic observation that in certain function fields, the Mobius function behaves like a quadratic Dirichlet character along certain arithmetic progressions. In principle, this reduces problems such as Chowla’s conjecture to problems about estimating sums of Dirichlet characters, for which more is known; but the task is still far from trivial.
“Bounds for sets with no polynomial progressions“, by Sarah Peluse. This paper can be viewed as part of a larger project to obtain quantitative density Ramsey theorems of Szemeredi type. For instance, Gowers famously established a relatively good quantitative bound for Szemeredi’s theorem that all dense subsets of integers contain arbitrarily long arithmetic progressions $a, a+r, \dots, a+(k-1)r$ . The corresponding question for polynomial progressions $a+P_1(r), \dots, a+P_k(r)$ is considered more difficult for a number of reasons. One of them is that dilation invariance is lost; a dilation of an arithmetic progression is again an arithmetic progression, but a dilation of a polynomial progression will in general not be a polynomial progression with the same polynomials $P_1,\dots,P_k$ . Another issue is that the ranges of the two parameters $a,r$ are now at different scales. Peluse gets around these difficulties in the case when all the polynomials $P_1,\dots,P_k$ have distinct degrees, which is in some sense the opposite case to that considered by Gowers (in particular, she avoids the need to obtain quantitative inverse theorems for high order Gowers norms; which was recently obtained in this integer setting by Manners but with bounds that are probably not strong enough to for the bounds in Peluse’s results, due to a degree lowering argument that is available in this case). To resolve the first difficulty one has to make all the estimates rather uniform in the coefficients of the polynomials $P_j$ , so that one can still run a density increment argument efficiently. To resolve the second difficulty one needs to find a quantitative concatenation theorem for Gowers uniformity norms. Many of these ideas were developed in previous papers of Peluse and Peluse-Prendiville in simpler settings.
“On blow up for the energy super critical defocusing non linear Schrödinger equations“, by Frank Merle, Pierre Raphael, Igor Rodnianski, and Jeremie Szeftel. This paper (when combined with two companion papers) resolves a long-standing problem as to whether finite time blowup occurs for the defocusing supercritical nonlinear Schrödinger equation (at least in certain dimensions and nonlinearities). I had a previous paper establishing a result like this if one “cheated” by replacing the nonlinear Schrodinger equation by a system of such equations, but remarkably they are able to tackle the original equation itself without any such cheating. Given the very analogous situation with Navier-Stokes, where again one can create finite time blowup by “cheating” and modifying the equation, it does raise hope that finite time blowup for the incompressible Navier-Stokes and Euler equations can be established… In fact the connection may not just be at the level of analogy; a surprising key ingredient in the proofs here is the observation that a certain blowup ansatz for the nonlinear Schrodinger equation is governed by solutions to the (compressible) Euler equation, and finite time blowup examples for the latter can be used to construct finite time blowup examples for the former.

Fourier uniformity of bounded multiplicative functions in short intervals on average

5 December, 2018 in math.NT, paper | Tags: Chowla conjecture, Kaisa Matomaki, Liouville function, Maksym Radziwill | by Terence Tao | 15 comments

Kaisa Matomäki, Maksym Radziwill, and I just uploaded to the arXiv our paper “Fourier uniformity of bounded multiplicative functions in short intervals on average“. This paper is the outcome of our attempts during the MSRI program in analytic number theory last year to attack the local Fourier uniformity conjecture for the Liouville function ${\lambda}$ . This conjecture generalises a landmark result of Matomäki and Radziwill, who show (among other things) that one has the asymptotic

$\displaystyle \int_X^{2X} |\sum_{x \leq n \leq x+H} \lambda(n)|\ dx = o(HX) \ \ \ \ \ (1)$

whenever ${X \rightarrow \infty}$ and ${H = H(X)}$ goes to infinity as ${X \rightarrow \infty}$ . Informally, this says that the Liouville function has small mean for almost all short intervals ${[x,x+H]}$ . The remarkable thing about this theorem is that there is no lower bound on how ${H}$ goes to infinity with ${X}$ ; one can take for instance ${H = \log\log\log X}$ . This lack of lower bound was crucial when I applied this result (or more precisely, a generalisation of this result to arbitrary non-pretentious bounded multiplicative functions) a few years ago to solve the Erdös discrepancy problem, as well as a logarithmically averaged two-point Chowla conjecture, for instance it implies that

$\displaystyle \sum_{n \leq X} \frac{\lambda(n) \lambda(n+1)}{n} = o(\log X).$

The local Fourier uniformity conjecture asserts the stronger asymptotic

$\displaystyle \int_X^{2X} \sup_{\alpha \in {\bf R}} |\sum_{x \leq n \leq x+H} \lambda(n) e(-\alpha n)|\ dx = o(HX) \ \ \ \ \ (2)$

under the same hypotheses on ${H}$ and ${X}$ . As I worked out in a previous paper, this conjecture would imply a logarithmically averaged three-point Chowla conjecture, implying for instance that

$\displaystyle \sum_{n \leq X} \frac{\lambda(n) \lambda(n+1) \lambda(n+2)}{n} = o(\log X).$

This particular bound also follows from some slightly different arguments of Joni Teräväinen and myself, but the implication would also work for other non-pretentious bounded multiplicative functions, whereas the arguments of Joni and myself rely more heavily on the specific properties of the Liouville function (in particular that ${\lambda(p)=-1}$ for all primes ${p}$ ).

There is also a higher order version of the local Fourier uniformity conjecture in which the linear phase ${{}e(-\alpha n)}$ is replaced with a polynomial phase such as ${e(-\alpha_d n^d - \dots - \alpha_1 n - \alpha_0)}$ , or more generally a nilsequence ${\overline{F(g(n) \Gamma)}}$ ; as shown in my previous paper, this conjecture implies (and is in fact equivalent to, after logarithmic averaging) a logarithmically averaged version of the full Chowla conjecture (not just the two-point or three-point versions), as well as a logarithmically averaged version of the Sarnak conjecture.

The main result of the current paper is to obtain some cases of the local Fourier uniformity conjecture:

Theorem 1 The asymptotic (2) is true when ${H = X^\theta}$ for a fixed ${\theta > 0}$ .

Previously this was known for ${\theta > 5/8}$ by the work of Zhan (who in fact proved the stronger pointwise assertion ${\sup_{\alpha \in {\bf R}} |\sum_{x \leq n \leq x+H} \lambda(n) e(-\alpha n)|= o(H)}$ for ${X \leq x \leq 2X}$ in this case). In a previous paper with Kaisa and Maksym, we also proved a weak version

$\displaystyle \sup_{\alpha \in {\bf R}} \int_X^{2X} |\sum_{x \leq n \leq x+H} \lambda(n) e(-\alpha n)|\ dx = o(HX) \ \ \ \ \ (3)$

of (2) for any ${H}$ growing arbitrarily slowly with ${X}$ ; this is stronger than (1) (and is in fact proven by a variant of the method) but significantly weaker than (2), because in the latter the worst-case ${\alpha}$ is permitted to depend on the ${x}$ parameter, whereas in (3) ${\alpha}$ must remain independent of ${x}$ .

Unfortunately, the restriction ${H = X^\theta}$ is not strong enough to give applications to Chowla-type conjectures (one would need something more like ${H = \log^\theta X}$ for this). However, it can still be used to control some sums that had not previously been manageable. For instance, a quick application of the circle method lets one use the above theorem to derive the asymptotic

$\displaystyle \sum_{h \leq H} \sum_{n \leq X} \lambda(n) \Lambda(n+h) \Lambda(n+2h) = o( H X )$

whenever ${H = X^\theta}$ for a fixed ${\theta > 0}$ , where ${\Lambda}$ is the von Mangoldt function. Amusingly, the seemingly simpler question of establishing the expected asymptotic for

$\displaystyle \sum_{h \leq H} \sum_{n \leq X} \Lambda(n+h) \Lambda(n+2h)$

is only known in the range ${\theta \geq 1/6}$ (from the work of Zaccagnini). Thus we have a rare example of a number theory sum that becomes easier to control when one inserts a Liouville function!

We now give an informal description of the strategy of proof of the theorem (though for numerous technical reasons, the actual proof deviates in some respects from the description given here). If (2) failed, then for many values of ${x \in [X,2X]}$ we would have the lower bound

$\displaystyle |\sum_{x \leq n \leq x+H} \lambda(n) e(-\alpha_x n)| \gg 1$

for some frequency ${\alpha_x \in{\bf R}}$ . We informally describe this correlation between ${\lambda(n)}$ and ${e(\alpha_x n)}$ by writing

$\displaystyle \lambda(n) \approx e(\alpha_x n) \ \ \ \ \ (4)$

for ${n \in [x,x+H]}$ (informally, one should view this as asserting that ${\lambda(n)}$ “behaves like” a constant multiple of ${e(\alpha_x n)}$ ). For sake of discussion, suppose we have this relationship for all ${x \in [X,2X]}$ , not just many.

As mentioned before, the main difficulty here is to understand how ${\alpha_x}$ varies with ${x}$ . As it turns out, the multiplicativity properties of the Liouville function place a significant constraint on this dependence. Indeed, if we let ${p}$ be a fairly small prime (e.g. of size ${H^\varepsilon}$ for some ${\varepsilon>0}$ ), and use the identity ${\lambda(np) = \lambda(n) \lambda(p) = - \lambda(n)}$ for the Liouville function to conclude (at least heuristically) from (4) that

$\displaystyle \lambda(n) \approx e(\alpha_x n p)$

for ${n \in [x/p, x/p + H/p]}$ . (In practice, we will have this sort of claim for many primes ${p}$ rather than all primes ${p}$ , after using tools such as the Turán-Kubilius inequality, but we ignore this distinction for this informal argument.)

Now let ${x, y \in [X,2X]}$ and ${p,q \sim P}$ be primes comparable to some fixed range ${P = H^\varepsilon}$ such that

$\displaystyle x/p = y/q + O( H/P). \ \ \ \ \ (5)$

Then we have both

$\displaystyle \lambda(n) \approx e(\alpha_x n p)$

and

$\displaystyle \lambda(n) \approx e(\alpha_y n q)$

on essentially the same range of ${n}$ (two nearby intervals of length ${\sim H/P}$ ). This suggests that the frequencies ${p \alpha_x}$ and ${q \alpha_y}$ should be close to each other modulo ${1}$ , in particular one should expect the relationship

$\displaystyle p \alpha_x = q \alpha_y + O( \frac{P}{H} ) \hbox{ mod } 1. \ \ \ \ \ (6)$

Comparing this with (5) one is led to the expectation that ${\alpha_x}$ should depend inversely on ${x}$ in some sense (for instance one can check that

$\displaystyle \alpha_x = T/x \ \ \ \ \ (7)$

would solve (6) if ${T = O( X / H^2 )}$ ; by Taylor expansion, this would correspond to a global approximation of the form ${\lambda(n) \approx n^{iT}}$ ). One now has a problem of an additive combinatorial flavour (or of a “local to global” flavour), namely to leverage the relation (6) to obtain global control on ${\alpha_x}$ that resembles (7).

A key obstacle in solving (6) efficiently is the fact that one only knows that ${p \alpha_x}$ and ${q \alpha_y}$ are close modulo ${1}$ , rather than close on the real line. One can start resolving this problem by the Chinese remainder theorem, using the fact that we have the freedom to shift (say) ${\alpha_y}$ by an arbitrary integer. After doing so, one can arrange matters so that one in fact has the relationship

$\displaystyle p \alpha_x = q \alpha_y + O( \frac{P}{H} ) \hbox{ mod } p \ \ \ \ \ (8)$

whenever ${x,y \in [X,2X]}$ and ${p,q \sim P}$ obey (5). (This may force ${\alpha_q}$ to become extremely large, on the order of ${\prod_{p \sim P} p}$ , but this will not concern us.)

Now suppose that we have ${y,y' \in [X,2X]}$ and primes ${q,q' \sim P}$ such that

$\displaystyle y/q = y'/q' + O(H/P). \ \ \ \ \ (9)$

For every prime ${p \sim P}$ , we can find an ${x}$ such that ${x/p}$ is within ${O(H/P)}$ of both ${y/q}$ and ${y'/q'}$ . Applying (8) twice we obtain

$\displaystyle p \alpha_x = q \alpha_y + O( \frac{P}{H} ) \hbox{ mod } p$

and

$\displaystyle p \alpha_x = q' \alpha_{y'} + O( \frac{P}{H} ) \hbox{ mod } p$

and thus by the triangle inequality we have

$\displaystyle q \alpha_y = q' \alpha_{y'} + O( \frac{P}{H} ) \hbox{ mod } p$

for all ${p \sim P}$ ; hence by the Chinese remainder theorem

$\displaystyle q \alpha_y = q' \alpha_{y'} + O( \frac{P}{H} ) \hbox{ mod } \prod_{p \sim P} p.$

In practice, in the regime ${H = X^\theta}$ that we are considering, the modulus ${\prod_{p \sim P} p}$ is so huge we can effectively ignore it (in the spirit of the Lefschetz principle); so let us pretend that we in fact have

$\displaystyle q \alpha_y = q' \alpha_{y'} + O( \frac{P}{H} ) \ \ \ \ \ (10)$

whenever ${y,y' \in [X,2X]}$ and ${q,q' \sim P}$ obey (9).

Now let ${k}$ be an integer to be chosen later, and suppose we have primes ${p_1,\dots,p_k,q_1,\dots,q_k \sim P}$ such that the difference

$\displaystyle q = |p_1 \dots p_k - q_1 \dots q_k|$

is small but non-zero. If ${k}$ is chosen so that

$\displaystyle P^k \approx \frac{X}{H}$

(where one is somewhat loose about what ${\approx}$ means) then one can then find real numbers ${x_1,\dots,x_k \sim X}$ such that

$\displaystyle \frac{x_j}{p_j} = \frac{x_{j+1}}{q_j} + O( \frac{H}{P} )$

for ${j=1,\dots,k}$ , with the convention that ${x_{k+1} = x_1}$ . We then have

$\displaystyle p_j \alpha_{x_j} = q_j \alpha_{x_{j+1}} + O( \frac{P}{H} )$

which telescopes to

$\displaystyle p_1 \dots p_k \alpha_{x_1} = q_1 \dots q_k \alpha_{x_1} + O( \frac{P^k}{H} )$

and thus

$\displaystyle q \alpha_{x_1} = O( \frac{P^k}{H} )$

and hence

$\displaystyle \alpha_{x_1} = O( \frac{P^k}{H} ) \approx O( \frac{X}{H^2} ).$

In particular, for each ${x \sim X}$ , we expect to be able to write

$\displaystyle \alpha_x = \frac{T_x}{x} + O( \frac{1}{H} )$

for some ${T_x = O( \frac{X^2}{H^2} )}$ . This quantity ${T_x}$ can vary with ${x}$ ; but from (10) and a short calculation we see that

$\displaystyle T_y = T_{y'} + O( \frac{X}{H} )$

whenever ${y, y' \in [X,2X]}$ obey (9) for some ${q,q' \sim P}$ .

Now imagine a “graph” in which the vertices are elements ${y}$ of ${[X,2X]}$ , and two elements ${y,y'}$ are joined by an edge if (9) holds for some ${q,q' \sim P}$ . Because of exponential sum estimates on ${\sum_{q \sim P} q^{it}}$ , this graph turns out to essentially be an “expander” in the sense that any two vertices ${y,y' \in [X,2X]}$ can be connected (in multiple ways) by fairly short paths in this graph (if one allows one to modify one of ${y}$ or ${y'}$ by ${O(H)}$ ). As a consequence, we can assume that this quantity ${T_y}$ is essentially constant in ${y}$ (cf. the application of the ergodic theorem in this previous blog post), thus we now have

$\displaystyle \alpha_x = \frac{T}{x} + O(\frac{1}{H} )$

for most ${x \in [X,2X]}$ and some ${T = O(X^2/H^2)}$ . By Taylor expansion, this implies that

$\displaystyle \lambda(n) \approx n^{iT}$

on ${[x,x+H]}$ for most ${x}$ , thus

$\displaystyle \int_X^{2X} |\sum_{x \leq n \leq x+H} \lambda(n) n^{-iT}|\ dx \gg HX.$

But this can be shown to contradict the Matomäki-Radziwill theorem (because the multiplicative function ${n \mapsto \lambda(n) n^{-iT}}$ is known to be non-pretentious).

The structure of correlations of multiplicative functions at almost all scales, with applications to the Chowla and Elliott conjectures

10 September, 2018 in math.DS, math.NT | Tags: Chowla conjecture, correlation, Elliott conjecture, Joni Teravainen | by Terence Tao | 5 comments

Joni Teräväinen and I have just uploaded to the arXiv our paper “The structure of correlations of multiplicative functions at almost all scales, with applications to the Chowla and Elliott conjectures“. This is a sequel to our previous paper that studied logarithmic correlations of the form

$\displaystyle f(a) := \lim^*_{x \rightarrow \infty} \frac{1}{\log \omega(x)} \sum_{x/\omega(x) \leq n \leq x} \frac{g_1(n+ah_1) \dots g_k(n+ah_k)}{n},$

where ${g_1,\dots,g_k}$ were bounded multiplicative functions, ${h_1,\dots,h_k \rightarrow \infty}$ were fixed shifts, ${1 \leq \omega(x) \leq x}$ was a quantity going off to infinity, and ${\lim^*}$ was a generalised limit functional. Our main technical result asserted that these correlations were necessarily the uniform limit of periodic functions ${f_i}$ . Furthermore, if ${g_1 \dots g_k}$ (weakly) pretended to be a Dirichlet character ${\chi}$ , then the ${f_i}$ could be chosen to be ${\chi}$ –isotypic in the sense that ${f_i(ab) = f_i(a) \chi(b)}$ whenever ${a,b}$ are integers with ${b}$ coprime to the periods of ${\chi}$ and ${f_i}$ ; otherwise, if ${g_1 \dots g_k}$ did not weakly pretend to be any Dirichlet character ${\chi}$ , then ${f}$ vanished completely. This was then used to verify several cases of the logarithmically averaged Elliott and Chowla conjectures.

The purpose of this paper was to investigate the extent to which the methods could be extended to non-logarithmically averaged settings. For our main technical result, we now considered the unweighted averages

$\displaystyle f_d(a) := \lim^*_{x \rightarrow \infty} \frac{1}{x/d} \sum_{n \leq x/d} g_1(n+ah_1) \dots g_k(n+ah_k),$

where ${d>1}$ is an additional parameter. Our main result was now as follows. If ${g_1 \dots g_k}$ did not weakly pretend to be a twisted Dirichlet character ${n \mapsto \chi(n) n^{it}}$ , then ${f_d(a)}$ converged to zero on (doubly logarithmic) average as ${d \rightarrow \infty}$ . If instead ${g_1 \dots g_k}$ did pretend to be such a twisted Dirichlet character, then ${f_d(a) d^{it}}$ converged on (doubly logarithmic) average to a limit ${f(a)}$ of ${\chi}$ -isotypic functions ${f_i}$ . Thus, roughly speaking, one has the approximation

$\displaystyle \lim^*_{x \rightarrow \infty} \frac{1}{x/d} \sum_{n \leq x/d} g_1(n+ah_1) \dots g_k(n+ah_k) \approx f(a) d^{-it}$

for most ${d}$ .

Informally, this says that at almost all scales ${x}$ (where “almost all” means “outside of a set of logarithmic density zero”), the non-logarithmic averages behave much like their logarithmic counterparts except for a possible additional twisting by an Archimedean character ${d \mapsto d^{it}}$ (which interacts with the Archimedean parameter ${d}$ in much the same way that the Dirichlet character ${\chi}$ interacts with the non-Archimedean parameter ${a}$ ). One consequence of this is that most of the recent results on the logarithmically averaged Chowla and Elliott conjectures can now be extended to their non-logarithmically averaged counterparts, so long as one excludes a set of exceptional scales ${x}$ of logarithmic density zero. For instance, the Chowla conjecture

$\displaystyle \lim_{x \rightarrow\infty} \frac{1}{x} \sum_{n \leq x} \lambda(n+h_1) \dots \lambda(n+h_k) = 0$

is now established for ${k}$ either odd or equal to ${2}$ , so long as one excludes an exceptional set of scales.

In the logarithmically averaged setup, the main idea was to combine two very different pieces of information on ${f(a)}$ . The first, coming from recent results in ergodic theory, was to show that ${f(a)}$ was well approximated in some sense by a nilsequence. The second was to use the “entropy decrement argument” to obtain an approximate isotopy property of the form

$\displaystyle f(a) g_1 \dots g_k(p)\approx f(ap)$

for “most” primes ${p}$ and integers ${a}$ . Combining the two facts, one eventually finds that only the almost periodic components of the nilsequence are relevant.

In the current situation, each ${a \mapsto f_d(a)}$ is approximated by a nilsequence, but the nilsequence can vary with ${d}$ (although there is some useful “Lipschitz continuity” of this nilsequence with respect to the ${d}$ parameter). Meanwhile, the entropy decrement argument gives an approximation basically of the form

$\displaystyle f_{dp}(a) g_1 \dots g_k(p)\approx f_d(ap)$

for “most” ${d,p,a}$ . The arguments then proceed largely as in the logarithmically averaged case. A key lemma to handle the dependence on the new parameter ${d}$ is the following cohomological statement: if one has a map ${\alpha: (0,+\infty) \rightarrow S^1}$ that was a quasimorphism in the sense that ${\alpha(xy) = \alpha(x) \alpha(y) + O(\varepsilon)}$ for all ${x,y \in (0,+\infty)}$ and some small ${\varepsilon}$ , then there exists a real number ${t}$ such that ${\alpha(x) = x^{it} + O(\varepsilon)}$ for all small ${\varepsilon}$ . This is achieved by applying a standard “cocycle averaging argument” to the cocycle ${(x,y) \mapsto \alpha(xy) \alpha(x)^{-1} \alpha(y)^{-1}}$ .

It would of course be desirable to not have the set of exceptional scales. We only know of one (implausible) scenario in which we can do this, namely when one has far fewer (in particular, subexponentially many) sign patterns for (say) the Liouville function than predicted by the Chowla conjecture. In this scenario (roughly analogous to the “Siegel zero” scenario in multiplicative number theory), the entropy of the Liouville sign patterns is so small that the entropy decrement argument becomes powerful enough to control all scales rather than almost all scales. On the other hand, this scenario seems to be self-defeating, in that it allows one to establish a large number of cases of the Chowla conjecture, and the full Chowla conjecture is inconsistent with having unusually few sign patterns. Still it hints that future work in this direction may need to split into “low entropy” and “high entropy” cases, in analogy to how many arguments in multiplicative number theory have to split into the “Siegel zero” and “no Siegel zero” cases.

The logarithmically averaged and non-logarithmically averaged Chowla conjectures

20 October, 2017 in expository, math.CO, math.DS, math.NT, math.PR | Tags: Chowla conjecture, second moment method | by Terence Tao | 8 comments

Let ${\lambda: {\bf N} \rightarrow \{-1,1\}}$ be the Liouville function, thus ${\lambda(n)}$ is defined to equal ${+1}$ when ${n}$ is the product of an even number of primes, and ${-1}$ when ${n}$ is the product of an odd number of primes. The Chowla conjecture asserts that ${\lambda}$ has the statistics of a random sign pattern, in the sense that

$\displaystyle \lim_{N \rightarrow \infty} \mathbb{E}_{n \leq N} \lambda(n+h_1) \dots \lambda(n+h_k) = 0 \ \ \ \ \ (1)$

for all ${k \geq 1}$ and all distinct natural numbers ${h_1,\dots,h_k}$ , where we use the averaging notation

$\displaystyle \mathbb{E}_{n \leq N} f(n) := \frac{1}{N} \sum_{n \leq N} f(n).$

For ${k=1}$ , this conjecture is equivalent to the prime number theorem (as discussed in this previous blog post), but the conjecture remains open for any ${k \geq 2}$ .

In recent years, it has been realised that one can make more progress on this conjecture if one works instead with the logarithmically averaged version

$\displaystyle \lim_{N \rightarrow \infty} \mathbb{E}_{n \leq N}^{\log} \lambda(n+h_1) \dots \lambda(n+h_k) = 0 \ \ \ \ \ (2)$

of the conjecture, where we use the logarithmic averaging notation

$\displaystyle \mathbb{E}_{n \leq N}^{\log} f(n) := \frac{\sum_{n \leq N} \frac{f(n)}{n}}{\sum_{n \leq N} \frac{1}{n}}.$

Using the summation by parts (or telescoping series) identity

$\displaystyle \sum_{n \leq N} \frac{f(n)}{n} = \sum_{M < N} \frac{1}{M(M+1)} (\sum_{n \leq M} f(n)) + \frac{1}{N} \sum_{n \leq N} f(n) \ \ \ \ \ (3)$

it is not difficult to show that the Chowla conjecture (1) for a given ${k,h_1,\dots,h_k}$ implies the logarithmically averaged conjecture (2). However, the converse implication is not at all clear. For instance, for ${k=1}$ , we have already mentioned that the Chowla conjecture

$\displaystyle \lim_{N \rightarrow \infty} \mathbb{E}_{n \leq N} \lambda(n) = 0$

is equivalent to the prime number theorem; but the logarithmically averaged analogue

$\displaystyle \lim_{N \rightarrow \infty} \mathbb{E}^{\log}_{n \leq N} \lambda(n) = 0$

is significantly easier to show (a proof with the Liouville function ${\lambda}$ replaced by the closely related Möbius function ${\mu}$ is given in this previous blog post). And indeed, significantly more is now known for the logarithmically averaged Chowla conjecture; in this paper of mine I had proven (2) for ${k=2}$ , and in this recent paper with Joni Teravainen, we proved the conjecture for all odd ${k}$ (with a different proof also given here).

In view of this emerging consensus that the logarithmically averaged Chowla conjecture was easier than the ordinary Chowla conjecture, it was thus somewhat of a surprise for me to read a recent paper of Gomilko, Kwietniak, and Lemanczyk who (among other things) established the following statement:

Theorem 1 Assume that the logarithmically averaged Chowla conjecture (2) is true for all ${k}$ . Then there exists a sequence ${N_i}$ going to infinity such that the Chowla conjecture (1) is true for all ${k}$ along that sequence, that is to say

$\displaystyle \lim_{N_i \rightarrow \infty} \mathbb{E}_{n \leq N_i} \lambda(n+h_1) \dots \lambda(n+h_k) = 0$

for all ${k}$ and all distinct ${h_1,\dots,h_k}$ .

This implication does not use any special properties of the Liouville function (other than that they are bounded), and in fact proceeds by ergodic theoretic methods, focusing in particular on the ergodic decomposition of invariant measures of a shift into ergodic measures. Ergodic methods have proven remarkably fruitful in understanding these sorts of number theoretic and combinatorial problems, as could already be seen by the ergodic theoretic proof of Szemerédi’s theorem by Furstenberg, and more recently by the work of Frantzikinakis and Host on Sarnak’s conjecture. (My first paper with Teravainen also uses ergodic theory tools.) Indeed, many other results in the subject were first discovered using ergodic theory methods.

On the other hand, many results in this subject that were first proven ergodic theoretically have since been reproven by more combinatorial means; my second paper with Teravainen is an instance of this. As it turns out, one can also prove Theorem 1 by a standard combinatorial (or probabilistic) technique known as the second moment method. In fact, one can prove slightly more:

Theorem 2 Let ${k}$ be a natural number. Assume that the logarithmically averaged Chowla conjecture (2) is true for ${2k}$ . Then there exists a set ${{\mathcal N}}$ of natural numbers of logarithmic density ${1}$ (that is, ${\lim_{N \rightarrow \infty} \mathbb{E}_{n \leq N}^{\log} 1_{n \in {\mathcal N}} = 1}$ ) such that

$\displaystyle \lim_{N \rightarrow \infty: N \in {\mathcal N}} \mathbb{E}_{n \leq N} \lambda(n+h_1) \dots \lambda(n+h_k) = 0$

for any distinct ${h_1,\dots,h_k}$ .

It is not difficult to deduce Theorem 1 from Theorem 2 using a diagonalisation argument. Unfortunately, the known cases of the logarithmically averaged Chowla conjecture ( ${k=2}$ and odd ${k}$ ) are currently insufficient to use Theorem 2 for any purpose other than to reprove what is already known to be true from the prime number theorem. (Indeed, the even cases of Chowla, in either logarithmically averaged or non-logarithmically averaged forms, seem to be far more powerful than the odd cases; see Remark 1.7 of this paper of myself and Teravainen for a related observation in this direction.)

We now sketch the proof of Theorem 2. For any distinct ${h_1,\dots,h_k}$ , we take a large number ${H}$ and consider the limiting the second moment

$\displaystyle \limsup_{N \rightarrow \infty} \mathop{\bf E}_{n \leq N}^{\log} |\mathop{\bf E}_{m \leq H} \lambda(n+m+h_1) \dots \lambda(n+m+h_k)|^2.$

We can expand this as

$\displaystyle \limsup_{N \rightarrow \infty} \mathop{\bf E}_{m,m' \leq H} \mathop{\bf E}_{n \leq N}^{\log} \lambda(n+m+h_1) \dots \lambda(n+m+h_k)$

$\displaystyle \lambda(n+m'+h_1) \dots \lambda(n+m'+h_k).$

If all the ${m+h_1,\dots,m+h_k,m'+h_1,\dots,m'+h_k}$ are distinct, the hypothesis (2) tells us that the inner averages goes to zero as ${N \rightarrow \infty}$ . The remaining averages are ${O(1)}$ , and there are ${O( k^2 )}$ of these averages. We conclude that

$\displaystyle \limsup_{N \rightarrow \infty} \mathop{\bf E}_{n \leq N}^{\log} |\mathop{\bf E}_{m \leq H} \lambda(n+m+h_1) \dots \lambda(n+m+h_k)|^2 \ll k^2 / H.$

By Markov’s inequality (and (3)), we conclude that for any fixed ${h_1,\dots,h_k, H}$ , there exists a set ${{\mathcal N}_{h_1,\dots,h_k,H}}$ of upper logarithmic density at least ${1-k/H^{1/2}}$ , thus

$\displaystyle \limsup_{N \rightarrow \infty} \mathbb{E}_{n \leq N}^{\log} 1_{n \in {\mathcal N}_{h_1,\dots,h_k,H}} \geq 1 - k/H^{1/2}$

such that

$\displaystyle \mathop{\bf E}_{n \leq N} |\mathop{\bf E}_{m \leq H} \lambda(n+m+h_1) \dots \lambda(n+m+h_k)|^2 \ll k / H^{1/2}.$

By deleting at most finitely many elements, we may assume that ${{\mathcal N}_{h_1,\dots,h_k,H}}$ consists only of elements of size at least ${H^2}$ (say).

For any ${H_0}$ , if we let ${{\mathcal N}_{h_1,\dots,h_k, \geq H_0}}$ be the union of ${{\mathcal N}_{h_1,\dots,h_k, H}}$ for ${H \geq H_0}$ , then ${{\mathcal N}_{h_1,\dots,h_k, \geq H_0}}$ has logarithmic density ${1}$ . By a diagonalisation argument (using the fact that the set of tuples ${(h_1,\dots,h_k)}$ is countable), we can then find a set ${{\mathcal N}}$ of natural numbers of logarithmic density ${1}$ , such that for every ${h_1,\dots,h_k,H_0}$ , every sufficiently large element of ${{\mathcal N}}$ lies in ${{\mathcal N}_{h_1,\dots,h_k,\geq H_0}}$ . Thus for every sufficiently large ${N}$ in ${{\mathcal N}}$ , one has

$\displaystyle \mathop{\bf E}_{n \leq N} |\mathop{\bf E}_{m \leq H} \lambda(n+m+h_1) \dots \lambda(n+m+h_k)|^2 \ll k / H^{1/2}.$

for some ${H \geq H_0}$ with ${N \geq H^2}$ . By Cauchy-Schwarz, this implies that

$\displaystyle \mathop{\bf E}_{n \leq N} \mathop{\bf E}_{m \leq H} \lambda(n+m+h_1) \dots \lambda(n+m+h_k) \ll k^{1/2} / H^{1/4};$

interchanging the sums and using ${N \geq H^2}$ and ${H \geq H_0}$ , this implies that

$\displaystyle \mathop{\bf E}_{n \leq N} \lambda(n+h_1) \dots \lambda(n+h_k) \ll k^{1/2} / H^{1/4} \leq k^{1/2} / H_0^{1/4}.$

We conclude on taking ${H_0}$ to infinity that

$\displaystyle \lim_{N \rightarrow \infty; N \in {\mathcal N}} \mathop{\bf E}_{n \leq N} \lambda(n+h_1) \dots \lambda(n+h_k) = 0$

as required.

Odd order cases of the logarithmically averaged Chowla conjecture

7 October, 2017 in math.NT, paper | Tags: Chowla conjecture, Joni Teravainen | by Terence Tao | 5 comments

Joni Teräväinen and I have just uploaded to the arXiv our paper “Odd order cases of the logarithmically averaged Chowla conjecture“, submitted to J. Numb. Thy. Bordeaux. This paper gives an alternate route to one of the main results of our previous paper, and more specifically reproves the asymptotic

$\displaystyle \sum_{n \leq x} \frac{\lambda(n+h_1) \dots \lambda(n+h_k)}{n} = o(\log x) \ \ \ \ \ (1)$

for all odd ${k}$ and all integers ${h_1,\dots,h_k}$ (that is to say, all the odd order cases of the logarithmically averaged Chowla conjecture). Our previous argument relies heavily on some deep ergodic theory results of Bergelson-Host-Kra, Leibman, and Le (and was applicable to more general multiplicative functions than the Liouville function ${\lambda}$ ); here we give a shorter proof that avoids ergodic theory (but instead requires the Gowers uniformity of the (W-tricked) von Mangoldt function, established in several papers of Ben Green, Tamar Ziegler, and myself). The proof follows the lines sketched in the previous blog post. In principle, due to the avoidance of ergodic theory, the arguments here have a greater chance to be made quantitative; however, at present the known bounds on the Gowers uniformity of the von Mangoldt function are qualitative, except at the ${U^2}$ level, which is unfortunate since the first non-trivial odd case ${k=3}$ requires quantitative control on the ${U^3}$ level. (But it may be possible to make the Gowers uniformity bounds for ${U^3}$ quantitative if one assumes GRH, although when one puts everything together, the actual decay rate obtained in (1) is likely to be poor.)

The structure of logarithmically averaged correlations of multiplicative functions, with applications to the Chowla and Elliott conjectures

9 August, 2017 in math.DS, math.NT, paper | Tags: Chowla conjecture, correspondence principle, Elliott conjecture, Joni Teravainen, Liouville function, multiplicative functions | by Terence Tao | 7 comments

Joni Teräväinen and I have just uploaded to the arXiv our paper “The structure of logarithmically averaged correlations of multiplicative functions, with applications to the Chowla and Elliott conjectures“, submitted to Duke Mathematical Journal. This paper builds upon my previous paper in which I introduced an “entropy decrement method” to prove the two-point (logarithmically averaged) cases of the Chowla and Elliott conjectures. A bit more specifically, I showed that

$\displaystyle \lim_{m \rightarrow \infty} \frac{1}{\log \omega_m} \sum_{x_m/\omega_m \leq n \leq x_m} \frac{g_0(n+h_0) g_1(n+h_1)}{n} = 0$

whenever ${1 \leq \omega_m \leq x_m}$ were sequences going to infinity, ${h_0,h_1}$ were distinct integers, and ${g_0,g_1: {\bf N} \rightarrow {\bf C}}$ were ${1}$ -bounded multiplicative functions which were non-pretentious in the sense that

$\displaystyle \liminf_{X \rightarrow \infty} \inf_{|t_j| \leq X} \sum_{p \leq X} \frac{1-\mathrm{Re}( g_j(p) \overline{\chi_j}(p) p^{it_j})}{p} = \infty \ \ \ \ \ (1)$

for all Dirichlet characters ${\chi_j}$ and for ${j=0,1}$ . Thus, for instance, one had the logarithmically averaged two-point Chowla conjecture

$\displaystyle \sum_{n \leq x} \frac{\lambda(n) \lambda(n+h)}{n} = o(\log x)$

for fixed any non-zero ${h}$ , where ${\lambda}$ was the Liouville function.

One would certainly like to extend these results to higher order correlations than the two-point correlations. This looks to be difficult (though perhaps not completely impossible if one allows for logarithmic averaging): in a previous paper I showed that achieving this in the context of the Liouville function would be equivalent to resolving the logarithmically averaged Sarnak conjecture, as well as establishing logarithmically averaged local Gowers uniformity of the Liouville function. However, in this paper we are able to avoid having to resolve these difficult conjectures to obtain partial results towards the (logarithmically averaged) Chowla and Elliott conjecture. For the Chowla conjecture, we can obtain all odd order correlations, in that

$\displaystyle \sum_{n \leq x} \frac{\lambda(n+h_1) \dots \lambda(n+h_k)}{n} = o(\log x) \ \ \ \ \ (2)$

for all odd ${k}$ and all integers ${h_1,\dots,h_k}$ (which, in the odd order case, are no longer required to be distinct). (Superficially, this looks like we have resolved “half of the cases” of the logarithmically averaged Chowla conjecture; but it seems the odd order correlations are significantly easier than the even order ones. For instance, because of the Katai-Bourgain-Sarnak-Ziegler criterion, one can basically deduce the odd order cases of (2) from the even order cases (after allowing for some dilations in the argument ${n}$ ).

For the more general Elliott conjecture, we can show that

$\displaystyle \lim_{m \rightarrow \infty} \frac{1}{\log \omega_m} \sum_{x_m/\omega_m \leq n \leq x_m} \frac{g_1(n+h_1) \dots g_k(n+h_k)}{n} = 0$

for any ${k}$ , any integers ${h_1,\dots,h_k}$ and any bounded multiplicative functions ${g_1,\dots,g_k}$ , unless the product ${g_1 \dots g_k}$ weakly pretends to be a Dirichlet character ${\chi}$ in the sense that

$\displaystyle \sum_{p \leq X} \frac{1 - \hbox{Re}( g_1 \dots g_k(p) \overline{\chi}(p)}{p} = o(\log\log X).$

This can be seen to imply (2) as a special case. Even when ${g_1,\dots,g_k}$ does pretend to be a Dirichlet character ${\chi}$ , we can still say something: if the limits

$\displaystyle f(a) := \lim_{m \rightarrow \infty} \frac{1}{\log \omega_m} \sum_{x_m/\omega_m \leq n \leq x_m} \frac{g_1(n+ah_1) \dots g_k(n+ah_k)}{n}$

exist for each ${a \in {\bf Z}}$ (which can be guaranteed if we pass to a suitable subsequence), then ${f}$ is the uniform limit of periodic functions ${f_i}$ , each of which is ${\chi}$ –isotypic in the sense that ${f_i(ab) = f_i(a) \chi(b)}$ whenever ${a,b}$ are integers with ${b}$ coprime to the periods of ${\chi}$ and ${f_i}$ . This does not pin down the value of any single correlation ${f(a)}$ , but does put significant constraints on how these correlations may vary with ${a}$ .

Among other things, this allows us to show that all ${16}$ possible length four sign patterns ${(\lambda(n+1),\dots,\lambda(n+4)) \in \{-1,+1\}^4}$ of the Liouville function occur with positive density, and all ${65}$ possible length four sign patterns ${(\mu(n+1),\dots,\mu(n+4)) \in \{-1,0,+1\}^4 \backslash \{-1,+1\}^4}$ occur with the conjectured logarithmic density. (In a previous paper with Matomaki and Radziwill, we obtained comparable results for length three patterns of Liouville and length two patterns of Möbius.)

To describe the argument, let us focus for simplicity on the case of the Liouville correlations

$\displaystyle f(a) := \lim_{X \rightarrow \infty} \frac{1}{\log X} \sum_{n \leq X} \frac{\lambda(n) \lambda(n+a) \dots \lambda(n+(k-1)a)}{n}, \ \ \ \ \ (3)$

assuming for sake of discussion that all limits exist. (In the paper, we instead use the device of generalised limits, as discussed in this previous post.) The idea is to combine together two rather different ways to control this function ${f}$ . The first proceeds by the entropy decrement method mentioned earlier, which roughly speaking works as follows. Firstly, we pick a prime ${p}$ and observe that ${\lambda(pn)=-\lambda(n)}$ for any ${n}$ , which allows us to rewrite (3) as

$\displaystyle (-1)^k f(a) = \lim_{X \rightarrow \infty} \frac{1}{\log X}$

$\displaystyle \sum_{n \leq X} \frac{\lambda(pn) \lambda(pn+ap) \dots \lambda(pn+(k-1)ap)}{n}.$

Making the change of variables ${n' = pn}$ , we obtain

$\displaystyle (-1)^k f(a) = \lim_{X \rightarrow \infty} \frac{1}{\log X}$

$\displaystyle \sum_{n' \leq pX} \frac{\lambda(n') \lambda(n'+ap) \dots \lambda(n'+(k-1)ap)}{n'} p 1_{p|n'}.$

The difference between ${n' \leq pX}$ and ${n' \leq X}$ is negligible in the limit (here is where we crucially rely on the log-averaging), hence

$\displaystyle (-1)^k f(a) = \lim_{X \rightarrow \infty} \frac{1}{\log X} \sum_{n \leq X} \frac{\lambda(n) \lambda(n+ap) \dots \lambda(n+(k-1)ap)}{n} p 1_{p|n}$

and thus by (3) we have

$\displaystyle (-1)^k f(a) = f(ap) + \lim_{X \rightarrow \infty} \frac{1}{\log X}$

$\displaystyle \sum_{n \leq X} \frac{\lambda(n) \lambda(n+ap) \dots \lambda(n+(k-1)ap)}{n} (p 1_{p|n}-1).$

The entropy decrement argument can be used to show that the latter limit is small for most ${p}$ (roughly speaking, this is because the factors ${p 1_{p|n}-1}$ behave like independent random variables as ${p}$ varies, so that concentration of measure results such as Hoeffding’s inequality can apply, after using entropy inequalities to decouple somewhat these random variables from the ${\lambda}$ factors). We thus obtain the approximate isotopy property

$\displaystyle (-1)^k f(a) \approx f(ap) \ \ \ \ \ (4)$

for most ${a}$ and ${p}$ .

On the other hand, by the Furstenberg correspondence principle (as discussed in these previous posts), it is possible to express ${f(a)}$ as a multiple correlation

$\displaystyle f(a) = \int_X g(x) g(T^a x) \dots g(T^{(k-1)a} x)\ d\mu(x)$

for some probability space ${(X,\mu)}$ equipped with a measure-preserving invertible map ${T: X \rightarrow X}$ . Using results of Bergelson-Host-Kra, Leibman, and Le, this allows us to obtain a decomposition of the form

$\displaystyle f(a) = f_1(a) + f_2(a) \ \ \ \ \ (5)$

where ${f_1}$ is a nilsequence, and ${f_2}$ goes to zero in density (even along the primes, or constant multiples of the primes). The original work of Bergelson-Host-Kra required ergodicity on ${X}$ , which is very definitely a hypothesis that is not available here; however, the later work of Leibman removed this hypothesis, and the work of Le refined the control on ${f_1}$ so that one still has good control when restricting to primes, or constant multiples of primes.

Ignoring the small error ${f_2(a)}$ , we can now combine (5) to conclude that

$\displaystyle f(a) \approx (-1)^k f_1(ap).$

Using the equidistribution theory of nilsequences (as developed in this previous paper of Ben Green and myself), one can break up ${f_1}$ further into a periodic piece ${f_0}$ and an “irrational” or “minor arc” piece ${f_3}$ . The contribution of the minor arc piece ${f_3}$ can be shown to mostly cancel itself out after dilating by primes ${p}$ and averaging, thanks to Vinogradov-type bilinear sum estimates (transferred to the primes). So we end up with

$\displaystyle f(a) \approx (-1)^k f_0(ap),$

which already shows (heuristically, at least) the claim that ${f}$ can be approximated by periodic functions ${f_0}$ which are isotopic in the sense that

$\displaystyle f_0(a) \approx (-1)^k f_0(ap).$

But if ${k}$ is odd, one can use Dirichlet’s theorem on primes in arithmetic progressions to restrict to primes ${p}$ that are ${1}$ modulo the period of ${f_0}$ , and conclude now that ${f_0}$ vanishes identically, which (heuristically, at least) gives (2).

The same sort of argument works to give the more general bounds on correlations of bounded multiplicative functions. But for the specific task of proving (2), we initially used a slightly different argument that avoids using the ergodic theory machinery of Bergelson-Host-Kra, Leibman, and Le, but replaces it instead with the Gowers uniformity norm theory used to count linear equations in primes. Basically, by averaging (4) in ${p}$ using the “ ${W}$ -trick”, as well as known facts about the Gowers uniformity of the von Mangoldt function, one can obtain an approximation of the form

$\displaystyle (-1)^k f(a) \approx {\bf E}_{b: (b,W)=1} f(ab)$

where ${b}$ ranges over a large range of integers coprime to some primorial ${W = \prod_{p \leq w} p}$ . On the other hand, by iterating (4) we have

$\displaystyle f(a) \approx f(apq)$

for most semiprimes ${pq}$ , and by again averaging over semiprimes one can obtain an approximation of the form

$\displaystyle f(a) \approx {\bf E}_{b: (b,W)=1} f(ab).$

For ${k}$ odd, one can combine the two approximations to conclude that ${f(a)=0}$ . (This argument is not given in the current paper, but we plan to detail it in a subsequent one.)

Furstenberg limits of the Liouville function

5 March, 2017 in expository, math.NT | Tags: Chowla conjecture, correspondence principle, Liouville function, randomness | by Terence Tao | 11 comments

Given a function ${f: {\bf N} \rightarrow \{-1,+1\}}$ on the natural numbers taking values in ${+1, -1}$ , one can invoke the Furstenberg correspondence principle to locate a measure preserving system ${T \circlearrowright (X, \mu)}$ – a probability space ${(X,\mu)}$ together with a measure-preserving shift ${T: X \rightarrow X}$ (or equivalently, a measure-preserving ${{\bf Z}}$ -action on ${(X,\mu)}$ ) – together with a measurable function (or “observable”) ${F: X \rightarrow \{-1,+1\}}$ that has essentially the same statistics as ${f}$ in the sense that

$\displaystyle \lim \inf_{N \rightarrow \infty} \frac{1}{N} \sum_{n=1}^N f(n+h_1) \dots f(n+h_k)$

$\displaystyle \leq \int_X F(T^{h_1} x) \dots F(T^{h_k} x)\ d\mu(x)$

$\displaystyle \leq \lim \sup_{N \rightarrow \infty} \frac{1}{N} \sum_{n=1}^N f(n+h_1) \dots f(n+h_k)$

for any integers ${h_1,\dots,h_k}$ . In particular, one has

$\displaystyle \int_X F(T^{h_1} x) \dots F(T^{h_k} x)\ d\mu(x) = \lim_{N \rightarrow \infty} \frac{1}{N} \sum_{n=1}^N f(n+h_1) \dots f(n+h_k) \ \ \ \ \ (1)$

whenever the limit on the right-hand side exists. We will refer to the system ${T \circlearrowright (X,\mu)}$ together with the designated function ${F}$ as a Furstenberg limit ot the sequence ${f}$ . These Furstenberg limits capture some, but not all, of the asymptotic behaviour of ${f}$ ; roughly speaking, they control the typical “local” behaviour of ${f}$ , involving correlations such as ${\frac{1}{N} \sum_{n=1}^N f(n+h_1) \dots f(n+h_k)}$ in the regime where ${h_1,\dots,h_k}$ are much smaller than ${N}$ . However, the control on error terms here is usually only qualitative at best, and one usually does not obtain non-trivial control on correlations in which the ${h_1,\dots,h_k}$ are allowed to grow at some significant rate with ${N}$ (e.g. like some power ${N^\theta}$ of ${N}$ ).

The correspondence principle is discussed in these previous blog posts. One way to establish the principle is by introducing a Banach limit ${p\!-\!\lim: \ell^\infty({\bf N}) \rightarrow {\bf R}}$ that extends the usual limit functional on the subspace of ${\ell^\infty({\bf N})}$ consisting of convergent sequences while still having operator norm one. Such functionals cannot be constructed explicitly, but can be proven to exist (non-constructively and non-uniquely) using the Hahn-Banach theorem; one can also use a non-principal ultrafilter here if desired. One can then seek to construct a system ${T \circlearrowright (X,\mu)}$ and a measurable function ${F: X \rightarrow \{-1,+1\}}$ for which one has the statistics

$\displaystyle \int_X F(T^{h_1} x) \dots F(T^{h_k} x)\ d\mu(x) = p\!-\!\lim_{N \rightarrow \infty} \frac{1}{N} \sum_{n=1}^N f(n+h_1) \dots f(n+h_k) \ \ \ \ \ (2)$

for all ${h_1,\dots,h_k}$ . One can explicitly construct such a system as follows. One can take ${X}$ to be the Cantor space ${\{-1,+1\}^{\bf Z}}$ with the product ${\sigma}$ -algebra and the shift

$\displaystyle T ( (x_n)_{n \in {\bf Z}} ) := (x_{n+1})_{n \in {\bf Z}}$

with the function ${F: X \rightarrow \{-1,+1\}}$ being the coordinate function at zero:

$\displaystyle F( (x_n)_{n \in {\bf Z}} ) := x_0$

(so in particular ${F( T^h (x_n)_{n \in {\bf Z}} ) = x_h}$ for any ${h \in {\bf Z}}$ ). The only thing remaining is to construct the invariant measure ${\mu}$ . In order to be consistent with (2), one must have

$\displaystyle \mu( \{ (x_n)_{n \in {\bf Z}}: x_{h_j} = \epsilon_j \forall 1 \leq j \leq k \} )$

$\displaystyle = p\!-\!\lim_{N \rightarrow \infty} \frac{1}{N} \sum_{n=1}^N 1_{f(n+h_1)=\epsilon_1} \dots 1_{f(n+h_k)=\epsilon_k}$

for any distinct integers ${h_1,\dots,h_k}$ and signs ${\epsilon_1,\dots,\epsilon_k}$ . One can check that this defines a premeasure on the Boolean algebra of ${\{-1,+1\}^{\bf Z}}$ defined by cylinder sets, and the existence of ${\mu}$ then follows from the Hahn-Kolmogorov extension theorem (or the closely related Kolmogorov extension theorem). One can then check that the correspondence (2) holds, and that ${\mu}$ is translation-invariant; the latter comes from the translation invariance of the (Banach-)Césaro averaging operation ${f \mapsto p\!-\!\lim_{N \rightarrow \infty} \frac{1}{N} \sum_{n=1}^N f(n)}$ . A variant of this construction shows that the Furstenberg limit is unique up to equivalence if and only if all the limits appearing in (1) actually exist.

One can obtain a slightly tighter correspondence by using a smoother average than the Césaro average. For instance, one can use the logarithmic Césaro averages ${\lim_{N \rightarrow \infty} \frac{1}{\log N}\sum_{n=1}^N \frac{f(n)}{n}}$ in place of the Césaro average ${\sum_{N \rightarrow \infty} \frac{1}{N} \sum_{n=1}^N f(n)}$ , thus one replaces (2) by

$\displaystyle \int_X F(T^{h_1} x) \dots F(T^{h_k} x)\ d\mu(x)$

$\displaystyle = p\!-\!\lim_{N \rightarrow \infty} \frac{1}{\log N} \sum_{n=1}^N \frac{f(n+h_1) \dots f(n+h_k)}{n}.$

Whenever the Césaro average of a bounded sequence ${f: {\bf N} \rightarrow {\bf R}}$ exists, then the logarithmic Césaro average exists and is equal to the Césaro average. Thus, a Furstenberg limit constructed using logarithmic Banach-Césaro averaging still obeys (1) for all ${h_1,\dots,h_k}$ when the right-hand side limit exists, but also obeys the more general assertion

$\displaystyle \int_X F(T^{h_1} x) \dots F(T^{h_k} x)\ d\mu(x)$

$\displaystyle = \lim_{N \rightarrow \infty} \frac{1}{\log N} \sum_{n=1}^N \frac{f(n+h_1) \dots f(n+h_k)}{n}$

whenever the limit of the right-hand side exists.

In a recent paper of Frantizinakis, the Furstenberg limits of the Liouville function ${\lambda}$ (with logarithmic averaging) were studied. Some (but not all) of the known facts and conjectures about the Liouville function can be interpreted in the Furstenberg limit. For instance, in a recent breakthrough result of Matomaki and Radziwill (discussed previously here), it was shown that the Liouville function exhibited cancellation on short intervals in the sense that

$\displaystyle \lim_{H \rightarrow \infty} \limsup_{X \rightarrow \infty} \frac{1}{X} \int_X^{2X} \frac{1}{H} |\sum_{x \leq n \leq x+H} \lambda(n)|\ dx = 0.$

In terms of Furstenberg limits of the Liouville function, this assertion is equivalent to the assertion that

$\displaystyle \lim_{H \rightarrow \infty} \int_X |\frac{1}{H} \sum_{h=1}^H F(T^h x)|\ d\mu(x) = 0$

for all Furstenberg limits ${T \circlearrowright (X,\mu), F}$ of Liouville (including those without logarithmic averaging). Invoking the mean ergodic theorem (discussed in this previous post), this assertion is in turn equivalent to the observable ${F}$ that corresponds to the Liouville function being orthogonal to the invariant factor ${L^\infty(X,\mu)^{\bf Z} = \{ g \in L^\infty(X,\mu): g \circ T = g \}}$ of ${X}$ ; equivalently, the first Gowers-Host-Kra seminorm ${\|F\|_{U^1(X)}}$ of ${F}$ (as defined for instance in this previous post) vanishes. The Chowla conjecture, which asserts that

$\displaystyle \lim_{N \rightarrow \infty} \frac{1}{N} \sum_{n=1}^N \lambda(n+h_1) \dots \lambda(n+h_k) = 0$

for all distinct integers ${h_1,\dots,h_k}$ , is equivalent to the assertion that all the Furstenberg limits of Liouville are equivalent to the Bernoulli system ( ${\{-1,+1\}^{\bf Z}}$ with the product measure arising from the uniform distribution on ${\{-1,+1\}}$ , with the shift ${T}$ and observable ${F}$ as before). Similarly, the logarithmically averaged Chowla conjecture

$\displaystyle \lim_{N \rightarrow \infty} \frac{1}{\log N} \sum_{n=1}^N \frac{\lambda(n+h_1) \dots \lambda(n+h_k)}{n} = 0$

is equivalent to the assertion that all the Furstenberg limits of Liouville with logarithmic averaging are equivalent to the Bernoulli system. Recently, I was able to prove the two-point version

$\displaystyle \lim_{N \rightarrow \infty} \frac{1}{\log N} \sum_{n=1}^N \frac{\lambda(n) \lambda(n+h)}{n} = 0 \ \ \ \ \ (3)$

of the logarithmically averaged Chowla conjecture, for any non-zero integer ${h}$ ; this is equivalent to the perfect strong mixing property

$\displaystyle \int_X F(x) F(T^h x)\ d\mu(x) = 0$

for any Furstenberg limit of Liouville with logarithmic averaging, and any ${h \neq 0}$ .

The situation is more delicate with regards to the Sarnak conjecture, which is equivalent to the assertion that

$\displaystyle \lim_{N \rightarrow \infty} \frac{1}{N} \sum_{n=1}^N \lambda(n) f(n) = 0$

for any zero-entropy sequence ${f: {\bf N} \rightarrow {\bf R}}$ (see this previous blog post for more discussion). Morally speaking, this conjecture should be equivalent to the assertion that any Furstenberg limit of Liouville is disjoint from any zero entropy system, but I was not able to formally establish an implication in either direction due to some technical issues regarding the fact that the Furstenberg limit does not directly control long-range correlations, only short-range ones. (There are however ergodic theoretic interpretations of the Sarnak conjecture that involve the notion of generic points; see this paper of El Abdalaoui, Lemancyk, and de la Rue.) But the situation is currently better with the logarithmically averaged Sarnak conjecture

$\displaystyle \lim_{N \rightarrow \infty} \frac{1}{\log N} \sum_{n=1}^N \frac{\lambda(n) f(n)}{n} = 0,$

as I was able to show that this conjecture was equivalent to the logarithmically averaged Chowla conjecture, and hence to all Furstenberg limits of Liouville with logarithmic averaging being Bernoulli; I also showed the conjecture was equivalent to local Gowers uniformity of the Liouville function, which is in turn equivalent to the function ${F}$ having all Gowers-Host-Kra seminorms vanishing in every Furstenberg limit with logarithmic averaging. In this recent paper of Frantzikinakis, this analysis was taken further, showing that the logarithmically averaged Chowla and Sarnak conjectures were in fact equivalent to the much milder seeming assertion that all Furstenberg limits with logarithmic averaging were ergodic.

Actually, the logarithmically averaged Furstenberg limits have more structure than just a ${{\bf Z}}$ -action on a measure preserving system ${(X,\mu)}$ with a single observable ${F}$ . Let ${Aff_+({\bf Z})}$ denote the semigroup of affine maps ${n \mapsto an+b}$ on the integers with ${a,b \in {\bf Z}}$ and ${a}$ positive. Also, let ${\hat {\bf Z}}$ denote the profinite integers (the inverse limit of the cyclic groups ${{\bf Z}/q{\bf Z}}$ ). Observe that ${Aff_+({\bf Z})}$ acts on ${\hat {\bf Z}}$ by taking the inverse limit of the obvious actions of ${Aff_+({\bf Z})}$ on ${{\bf Z}/q{\bf Z}}$ .

Proposition 1 (Enriched logarithmically averaged Furstenberg limit of Liouville) Let ${p\!-\!\lim}$ be a Banach limit. Then there exists a probability space ${(X,\mu)}$ with an action ${\phi \mapsto T^\phi}$ of the affine semigroup ${Aff_+({\bf Z})}$ , as well as measurable functions ${F: X \rightarrow \{-1,+1\}}$ and ${M: X \rightarrow \hat {\bf Z}}$ , with the following properties:

(i) (Affine Furstenberg limit) For any ${\phi_1,\dots,\phi_k \in Aff_+({\bf Z})}$ , and any congruence class ${a\ (q)}$ , one has
$\displaystyle p\!-\!\lim_{N \rightarrow \infty} \frac{1}{\log N} \sum_{n=1}^N \frac{\lambda(\phi_1(n)) \dots \lambda(\phi_k(n)) 1_{n = a\ (q)}}{n}$

$\displaystyle = \int_X F( T^{\phi_1}(x) ) \dots F( T^{\phi_k}(x) ) 1_{M(x) = a\ (q)}\ d\mu(x).$

(ii) (Equivariance of ${M}$ ) For any ${\phi \in Aff_+({\bf Z})}$ , one has
$\displaystyle M( T^\phi(x) ) = \phi( M(x) )$

for ${\mu}$ -almost every ${x \in X}$ .

(iii) (Multiplicativity at fixed primes) For any prime ${p}$ , one has
$\displaystyle F( T^{p\cdot} x ) = - F(x)$

for ${\mu}$ -almost every ${x \in X}$ , where ${p \cdot \in Aff_+({\bf Z})}$ is the dilation map ${n \mapsto pn}$ .

(iv) (Measure pushforward) If ${\phi \in Aff_+({\bf Z})}$ is of the form ${\phi(n) = an+b}$ and ${S_\phi \subset X}$ is the set ${S_\phi = \{ x \in X: M(x) \in \phi(\hat {\bf Z}) \}}$ , then the pushforward ${T^\phi_* \mu}$ of ${\mu}$ by ${\phi}$ is equal to ${a \mu\downharpoonright_{S_\phi}}$ , that is to say one has
$\displaystyle \mu( (T^\phi)^{-1}(E) ) = a \mu( E \cap S_\phi )$

for every measurable ${E \subset X}$ .

Note that ${{\bf Z}}$ can be viewed as the subgroup of ${Aff_+({\bf Z})}$ consisting of the translations ${n \mapsto n + b}$ . If one only keeps the ${{\bf Z}}$ -portion of the ${Aff_+({\bf Z})}$ action and forgets the rest (as well as the function ${M}$ ) then the action becomes measure-preserving, and we recover an ordinary Furstenberg limit with logarithmic averaging. However, the additional structure here can be quite useful; for instance, one can transfer the proof of (3) to this setting, which we sketch below the fold, after proving the proposition.

The observable ${M}$ , roughly speaking, means that points ${x}$ in the Furstenberg limit ${X}$ constructed by this proposition are still “virtual integers” in the sense that one can meaningfully compute the residue class of ${x}$ modulo any natural number modulus ${q}$ , by first applying ${M}$ and then reducing mod ${q}$ . The action of ${Aff_+({\bf Z})}$ means that one can also meaningfully multiply ${x}$ by any natural number, and translate it by any integer. As with other applications of the correspondence principle, the main advantage of moving to this more “virtual” setting is that one now acquires a probability measure ${\mu}$ , so that the tools of ergodic theory can be readily applied.

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Equivalence of the logarithmically averaged Chowla and Sarnak conjectures

16 May, 2016 in math.NT, paper | Tags: Chowla conjecture, entropy decrement argument, Sarnak conjecture | by Terence Tao | 7 comments

I’ve just uploaded to the arXiv my paper “Equivalence of the logarithmically averaged Chowla and Sarnak conjectures“, submitted to the Festschrift “Number Theory – Diophantine problems, uniform distribution and applications” in honour of Robert F. Tichy. This paper is a spinoff of my previous paper establishing a logarithmically averaged version of the Chowla (and Elliott) conjectures in the two-point case. In that paper, the estimate

$\displaystyle \sum_{n \leq x} \frac{\lambda(n) \lambda(n+h)}{n} = o( \log x )$

as ${x \rightarrow \infty}$ was demonstrated, where ${h}$ was any positive integer and ${\lambda}$ denoted the Liouville function. The proof proceeded using a method I call the “entropy decrement argument”, which ultimately reduced matters to establishing a bound of the form

$\displaystyle \sum_{n \leq x} \frac{|\sum_{h \leq H} \lambda(n+h) e( \alpha h)|}{n} = o( H \log x )$

whenever ${H}$ was a slowly growing function of ${x}$ . This was in turn established in a previous paper of Matomaki, Radziwill, and myself, using the recent breakthrough of Matomaki and Radziwill.

It is natural to see to what extent the arguments can be adapted to attack the higher-point cases of the logarithmically averaged Chowla conjecture (ignoring for this post the more general Elliott conjecture for other bounded multiplicative functions than the Liouville function). That is to say, one would like to prove that

$\displaystyle \sum_{n \leq x} \frac{\lambda(n+h_1) \dots \lambda(n+h_k)}{n} = o( \log x )$

as ${x \rightarrow \infty}$ for any fixed distinct integers ${h_1,\dots,h_k}$ . As it turns out (and as is detailed in the current paper), the entropy decrement argument extends to this setting (after using some known facts about linear equations in primes), and allows one to reduce the above estimate to an estimate of the form

$\displaystyle \sum_{n \leq x} \frac{1}{n} \| \lambda \|_{U^d[n, n+H]} = o( \log x )$

for ${H}$ a slowly growing function of ${x}$ and some fixed ${d}$ (in fact we can take ${d=k-1}$ for ${k \geq 3}$ ), where ${U^d}$ is the (normalised) local Gowers uniformity norm. (In the case ${k=3}$ , ${d=2}$ , this becomes the Fourier-uniformity conjecture discussed in this previous post.) If one then applied the (now proven) inverse conjecture for the Gowers norms, this estimate is in turn equivalent to the more complicated looking assertion

$\displaystyle \sum_{n \leq x} \frac{1}{n} \sup |\sum_{h \leq H} \lambda(n+h) F( g^h x )| = o( \log x ) \ \ \ \ \ (1)$

where the supremum is over all possible choices of nilsequences ${h \mapsto F(g^h x)}$ of controlled step and complexity (see the paper for definitions of these terms).

The main novelty in the paper (elaborating upon a previous comment I had made on this blog) is to observe that this latter estimate in turn follows from the logarithmically averaged form of Sarnak’s conjecture (discussed in this previous post), namely that

$\displaystyle \sum_{n \leq x} \frac{1}{n} \lambda(n) F( T^n x )= o( \log x )$

whenever ${n \mapsto F(T^n x)}$ is a zero entropy (i.e. deterministic) sequence. Morally speaking, this follows from the well-known fact that nilsequences have zero entropy, but the presence of the supremum in (1) means that we need a little bit more; roughly speaking, we need the class of nilsequences of a given step and complexity to have “uniformly zero entropy” in some sense.

On the other hand, it was already known (see previous post) that the Chowla conjecture implied the Sarnak conjecture, and similarly for the logarithmically averaged form of the two conjectures. Putting all these implications together, we obtain the pleasant fact that the logarithmically averaged Sarnak and Chowla conjectures are equivalent, which is the main result of the current paper. There have been a large number of special cases of the Sarnak conjecture worked out (when the deterministic sequence involved came from a special dynamical system), so these results can now also be viewed as partial progress towards the Chowla conjecture also (at least with logarithmic averaging). However, my feeling is that the full resolution of these conjectures will not come from these sorts of special cases; instead, conjectures like the Fourier-uniformity conjecture in this previous post look more promising to attack.

It would also be nice to get rid of the pesky logarithmic averaging, but this seems to be an inherent requirement of the entropy decrement argument method, so one would probably have to find a way to avoid that argument if one were to remove the log averaging.

A conjectural local Fourier-uniformity of the Liouville function

2 December, 2015 in expository, math.NT, question | Tags: Chowla conjecture, Gowers uniformity norm, Liouville function | by Terence Tao | 22 comments

Let ${\lambda}$ denote the Liouville function. The prime number theorem is equivalent to the estimate

$\displaystyle \sum_{n \leq x} \lambda(n) = o(x)$

as ${x \rightarrow \infty}$ , that is to say that ${\lambda}$ exhibits cancellation on large intervals such as ${[1,x]}$ . This result can be improved to give cancellation on shorter intervals. For instance, using the known zero density estimates for the Riemann zeta function, one can establish that

$\displaystyle \int_X^{2X} |\sum_{x \leq n \leq x+H} \lambda(n)|\ dx = o( HX ) \ \ \ \ \ (1)$

as ${X \rightarrow \infty}$ if ${X^{1/6+\varepsilon} \leq H \leq X}$ for some fixed ${\varepsilon>0}$ ; I believe this result is due to Ramachandra (see also Exercise 21 of this previous blog post), and in fact one could obtain a better error term on the right-hand side that for instance gained an arbitrary power of ${\log X}$ . On the Riemann hypothesis (or the weaker density hypothesis), it was known that the ${X^{1/6+\varepsilon}}$ could be lowered to ${X^\varepsilon}$ .

Early this year, there was a major breakthrough by Matomaki and Radziwill, who (among other things) showed that the asymptotic (1) was in fact valid for any ${H = H(X)}$ with ${H \leq X}$ that went to infinity as ${X \rightarrow \infty}$ , thus yielding cancellation on extremely short intervals. This has many further applications; for instance, this estimate, or more precisely its extension to other “non-pretentious” bounded multiplicative functions, was a key ingredient in my recent solution of the Erdös discrepancy problem, as well as in obtaining logarithmically averaged cases of Chowla’s conjecture, such as

$\displaystyle \sum_{n \leq x} \frac{\lambda(n) \lambda(n+1)}{n} = o(\log x). \ \ \ \ \ (2)$

It is of interest to twist the above estimates by phases such as the linear phase ${n \mapsto e(\alpha n) := e^{2\pi i \alpha n}}$ . In 1937, Davenport showed that

$\displaystyle \sup_\alpha |\sum_{n \leq x} \lambda(n) e(\alpha n)| \ll_A x \log^{-A} x$

which of course improves the prime number theorem. Recently with Matomaki and Radziwill, we obtained a common generalisation of this estimate with (1), showing that

$\displaystyle \sup_\alpha \int_X^{2X} |\sum_{x \leq n \leq x+H} \lambda(n) e(\alpha n)|\ dx = o(HX) \ \ \ \ \ (3)$

as ${X \rightarrow \infty}$ , for any ${H = H(X) \leq X}$ that went to infinity as ${X \rightarrow \infty}$ . We were able to use this estimate to obtain an averaged form of Chowla’s conjecture.

In that paper, we asked whether one could improve this estimate further by moving the supremum inside the integral, that is to say to establish the bound

$\displaystyle \int_X^{2X} \sup_\alpha |\sum_{x \leq n \leq x+H} \lambda(n) e(\alpha n)|\ dx = o(HX) \ \ \ \ \ (4)$

as ${X \rightarrow \infty}$ , for any ${H = H(X) \leq X}$ that went to infinity as ${X \rightarrow \infty}$ . This bound is asserting that ${\lambda}$ is locally Fourier-uniform on most short intervals; it can be written equivalently in terms of the “local Gowers ${U^2}$ norm” as

$\displaystyle \int_X^{2X} \sum_{1 \leq a \leq H} |\sum_{x \leq n \leq x+H} \lambda(n) \lambda(n+a)|^2\ dx = o( H^3 X )$

from which one can see that this is another averaged form of Chowla’s conjecture (stronger than the one I was able to prove with Matomaki and Radziwill, but a consequence of the unaveraged Chowla conjecture). If one inserted such a bound into the machinery I used to solve the Erdös discrepancy problem, it should lead to further averaged cases of Chowla’s conjecture, such as

$\displaystyle \sum_{n \leq x} \frac{\lambda(n) \lambda(n+1) \lambda(n+2)}{n} = o(\log x), \ \ \ \ \ (5)$

though I have not fully checked the details of this implication. It should also have a number of new implications for sign patterns of the Liouville function, though we have not explored these in detail yet.

One can write (4) equivalently in the form

$\displaystyle \int_X^{2X} \sum_{x \leq n \leq x+H} \lambda(n) e( \alpha(x) n + \beta(x) )\ dx = o(HX) \ \ \ \ \ (6)$

uniformly for all ${x}$ -dependent phases ${\alpha(x), \beta(x)}$ . In contrast, (3) is equivalent to the subcase of (6) when the linear phase coefficient ${\alpha(x)}$ is independent of ${x}$ . This dependency of ${\alpha(x)}$ on ${x}$ seems to necessitate some highly nontrivial additive combinatorial analysis of the function ${x \mapsto \alpha(x)}$ in order to establish (4) when ${H}$ is small. To date, this analysis has proven to be elusive, but I would like to record what one can do with more classical methods like Vaughan’s identity, namely:

Proposition 1 The estimate (4) (or equivalently (6)) holds in the range ${X^{2/3+\varepsilon} \leq H \leq X}$ for any fixed ${\varepsilon>0}$ . (In fact one can improve the right-hand side by an arbitrary power of ${\log X}$ in this case.)

The values of ${H}$ in this range are far too large to yield implications such as new cases of the Chowla conjecture, but it appears that the ${2/3}$ exponent is the limit of “classical” methods (at least as far as I was able to apply them), in the sense that one does not do any combinatorial analysis on the function ${x \mapsto \alpha(x)}$ , nor does one use modern equidistribution results on “Type III sums” that require deep estimates on Kloosterman-type sums. The latter may shave a little bit off of the ${2/3}$ exponent, but I don’t see how one would ever hope to go below ${1/2}$ without doing some non-trivial combinatorics on the function ${x \mapsto \alpha(x)}$ . UPDATE: I have come across this paper of Zhan which uses mean-value theorems for L-functions to lower the ${2/3}$ exponent to ${5/8}$ .

Let me now sketch the proof of the proposition, omitting many of the technical details. We first remark that known estimates on sums of the Liouville function (or similar functions such as the von Mangoldt function) in short arithmetic progressions, based on zero-density estimates for Dirichlet ${L}$ -functions, can handle the “major arc” case of (4) (or (6)) where ${\alpha}$ is restricted to be of the form ${\alpha = \frac{a}{q} + O( X^{-1/6-\varepsilon} )}$ for ${q = O(\log^{O(1)} X)}$ (the exponent here being of the same numerology as the ${X^{1/6+\varepsilon}}$ exponent in the classical result of Ramachandra, tied to the best zero density estimates currently available); for instance a modification of the arguments in this recent paper of Koukoulopoulos would suffice. Thus we can restrict attention to “minor arc” values of ${\alpha}$ (or ${\alpha(x)}$ , using the interpretation of (6)).

Next, one breaks up ${\lambda}$ (or the closely related Möbius function) into Dirichlet convolutions using one of the standard identities (e.g. Vaughan’s identity or Heath-Brown’s identity), as discussed for instance in this previous post (which is focused more on the von Mangoldt function, but analogous identities exist for the Liouville and Möbius functions). The exact choice of identity is not terribly important, but the upshot is that ${\lambda(n)}$ can be decomposed into ${\log^{O(1)} X}$ terms, each of which is either of the “Type I” form

$\displaystyle \sum_{d \sim D; m \sim M: dm=n} a_d$

for some coefficients ${a_d}$ that are roughly of logarithmic size on the average, and scales ${D, M}$ with ${D \ll X^{2/3}}$ and ${DM \sim X}$ , or else of the “Type II” form

$\displaystyle \sum_{d \sim D; m \sim M: dm=n} a_d b_m$

for some coefficients ${a_d, b_m}$ that are roughly of logarithmic size on the average, and scales ${D,M}$ with ${X^{1/3} \ll D,M \ll X^{2/3}}$ and ${DM \sim X}$ . As discussed in the previous post, the ${2/3}$ exponent is a natural barrier in these identities if one is unwilling to also consider “Type III” type terms which are roughly of the shape of the third divisor function ${\tau_3(n) := \sum_{d_1d_2d_3=1} 1}$ .

A Type I sum makes a contribution to ${ \sum_{x \leq n \leq x+H} \lambda(n) e( \alpha(x) n + \beta(x) )}$ that can be bounded (via Cauchy-Schwarz) in terms of an expression such as

$\displaystyle \sum_{d \sim D} | \sum_{x/d \leq m \leq x/d+H/d} e(\alpha(x) dm )|^2.$

The inner sum exhibits a lot of cancellation unless ${\alpha(x) d}$ is within ${O(D/H)}$ of an integer. (Here, “a lot” should be loosely interpreted as “gaining many powers of ${\log X}$ over the trivial bound”.) Since ${H}$ is significantly larger than ${D}$ , standard Vinogradov-type manipulations (see e.g. Lemma 13 of these previous notes) show that this bad case occurs for many ${d}$ only when ${\alpha}$ is “major arc”, which is the case we have specifically excluded. This lets us dispose of the Type I contributions.

A Type II sum makes a contribution to ${ \sum_{x \leq n \leq x+H} \lambda(n) e( \alpha(x) n + \beta(x) )}$ roughly of the form

$\displaystyle \sum_{d \sim D} | \sum_{x/d \leq m \leq x/d+H/d} b_m e(\alpha(x) dm)|.$

We can break this up into a number of sums roughly of the form

$\displaystyle \sum_{d = d_0 + O( H / M )} | \sum_{x/d_0 \leq m \leq x/d_0 + H/D} b_m e(\alpha(x) dm)|$

for ${d_0 \sim D}$ ; note that the ${d}$ range is non-trivial because ${H}$ is much larger than ${M}$ . Applying the usual bilinear sum Cauchy-Schwarz methods (e.g. Theorem 14 of these notes) we conclude that there is a lot of cancellation unless one has ${\alpha(x) = a/q + O( \frac{X \log^{O(1)} X}{H^2} )}$ for some ${q = O(\log^{O(1)} X)}$ . But with ${H \geq X^{2/3+\varepsilon}}$ , ${X \log^{O(1)} X/H^2}$ is well below the threshold ${X^{-1/6-\varepsilon}}$ for the definition of major arc, so we can exclude this case and obtain the required cancellation.

A small remark on the Elliott conjecture

30 October, 2015 in expository, math.NT | Tags: Chowla conjecture, Elliott conjecture, Liouville function | by Terence Tao | 4 comments

The Chowla conjecture asserts, among other things, that one has the asymptotic

$\displaystyle \frac{1}{X} \sum_{n \leq X} \lambda(n+h_1) \dots \lambda(n+h_k) = o(1)$

as ${X \rightarrow \infty}$ for any distinct integers ${h_1,\dots,h_k}$ , where ${\lambda}$ is the Liouville function. (The usual formulation of the conjecture also allows one to consider more general linear forms ${a_i n + b_i}$ than the shifts ${n+h_i}$ , but for sake of discussion let us focus on the shift case.) This conjecture remains open for ${k \geq 2}$ , though there are now some partial results when one averages either in ${x}$ or in the ${h_1,\dots,h_k}$ , as discussed in this recent post.

A natural generalisation of the Chowla conjecture is the Elliott conjecture. Its original formulation was basically as follows: one had

$\displaystyle \frac{1}{X} \sum_{n \leq X} g_1(n+h_1) \dots g_k(n+h_k) = o(1) \ \ \ \ \ (1)$

whenever ${g_1,\dots,g_k}$ were bounded completely multiplicative functions and ${h_1,\dots,h_k}$ were distinct integers, and one of the ${g_i}$ was “non-pretentious” in the sense that

$\displaystyle \sum_p \frac{1 - \hbox{Re}( g_i(p) \overline{\chi(p)} p^{-it})}{p} = +\infty \ \ \ \ \ (2)$

for all Dirichlet characters ${\chi}$ and real numbers ${t}$ . It is easy to see that some condition like (2) is necessary; for instance if ${g(n) := \chi(n) n^{it}}$ and ${\chi}$ has period ${q}$ then ${\frac{1}{X} \sum_{n \leq X} g(n+q) \overline{g(n)}}$ can be verified to be bounded away from zero as ${X \rightarrow \infty}$ .

In a previous paper with Matomaki and Radziwill, we provided a counterexample to the original formulation of the Elliott conjecture, and proposed that (2) be replaced with the stronger condition

$\displaystyle \inf_{|t| \leq X} \sum_{p \leq X} \frac{1 - \hbox{Re}( g_i(p) \overline{\chi(p)} p^{-it})}{p} \rightarrow +\infty \ \ \ \ \ (3)$

as ${X \rightarrow \infty}$ for any Dirichlet character ${\chi}$ . To support this conjecture, we proved an averaged and non-asymptotic version of this conjecture which roughly speaking showed a bound of the form

$\displaystyle \frac{1}{H^k} \sum_{h_1,\dots,h_k \leq H} |\frac{1}{X} \sum_{n \leq X} g_1(n+h_1) \dots g_k(n+h_k)| \leq \varepsilon$

whenever ${H}$ was an arbitrarily slowly growing function of ${X}$ , ${X}$ was sufficiently large (depending on ${\varepsilon,k}$ and the rate at which ${H}$ grows), and one of the ${g_i}$ obeyed the condition

$\displaystyle \inf_{|t| \leq AX} \sum_{p \leq X} \frac{1 - \hbox{Re}( g_i(p) \overline{\chi(p)} p^{-it})}{p} \geq A \ \ \ \ \ (4)$

for some ${A}$ that was sufficiently large depending on ${k,\varepsilon}$ , and all Dirichlet characters ${\chi}$ of period at most ${A}$ . As further support of this conjecture, I recently established the bound

$\displaystyle \frac{1}{\log \omega} |\sum_{X/\omega \leq n \leq X} \frac{g_1(n+h_1) g_2(n+h_2)}{n}| \leq \varepsilon$

under the same hypotheses, where ${\omega}$ is an arbitrarily slowly growing function of ${X}$ .

In view of these results, it is tempting to conjecture that the condition (4) for one of the ${g_i}$ should be sufficient to obtain the bound

$\displaystyle |\frac{1}{X} \sum_{n \leq X} g_1(n+h_1) \dots g_k(n+h_k)| \leq \varepsilon$

when ${A}$ is large enough depending on ${k,\varepsilon}$ . This may well be the case for ${k=2}$ . However, the purpose of this blog post is to record a simple counterexample for ${k>2}$ . Let’s take ${k=3}$ for simplicity. Let ${t_0}$ be a quantity much larger than ${X}$ but much smaller than ${X^2}$ (e.g. ${t = X^{3/2}}$ ), and set

$\displaystyle g_1(n) := n^{it_0}; \quad g_2(n) := n^{-2it_0}; \quad g_3(n) := n^{it_0}.$

For ${X/2 \leq n \leq X}$ , Taylor expansion gives

$\displaystyle (n+1)^{it} = n^{it_0} \exp( i t_0 / n ) + o(1)$

and

$\displaystyle (n+2)^{it} = n^{it_0} \exp( 2 i t_0 / n ) + o(1)$

and hence

$\displaystyle g_1(n) g_2(n+1) g_3(n+2) = 1 + o(1)$

and hence

$\displaystyle |\frac{1}{X} \sum_{X/2 \leq n \leq X} g_1(n) g_2(n+1) g_3(n+2)| \gg 1.$

On the other hand one can easily verify that all of the ${g_1,g_2,g_3}$ obey (4) (the restriction ${|t| \leq AX}$ there prevents ${t}$ from getting anywhere close to ${t_0}$ ). So it seems the correct non-asymptotic version of the Elliott conjecture is the following:

Conjecture 1 (Non-asymptotic Elliott conjecture) Let ${k}$ be a natural number, and let ${h_1,\dots,h_k}$ be integers. Let ${\varepsilon > 0}$ , let ${A}$ be sufficiently large depending on ${k,\varepsilon,h_1,\dots,h_k}$ , and let ${X}$ be sufficiently large depending on ${k,\varepsilon,h_1,\dots,h_k,A}$ . Let ${g_1,\dots,g_k}$ be bounded multiplicative functions such that for some ${1 \leq i \leq k}$ , one has

$\displaystyle \inf_{|t| \leq AX^{k-1}} \sum_{p \leq X} \frac{1 - \hbox{Re}( g_i(p) \overline{\chi(p)} p^{-it})}{p} \geq A$

for all Dirichlet characters ${\chi}$ of conductor at most ${A}$ . Then

$\displaystyle |\frac{1}{X} \sum_{n \leq X} g_1(n+h_1) \dots g_k(n+h_k)| \leq \varepsilon.$

The ${k=1}$ case of this conjecture follows from the work of Halasz; in my recent paper a logarithmically averaged version of the ${k=2}$ case of this conjecture is established. The requirement to take ${t}$ to be as large as ${A X^{k-1}}$ does not emerge in the averaged Elliott conjecture in my previous paper with Matomaki and Radziwill; it thus seems that this averaging has concealed some of the subtler features of the Elliott conjecture. (However, this subtlety does not seem to affect the asymptotic version of the conjecture formulated in that paper, in which the hypothesis is of the form (3), and the conclusion is of the form (1).)

A similar subtlety arises when trying to control the maximal integral

$\displaystyle \frac{1}{X} \int_X^{2X} \sup_\alpha \frac{1}{H} |\sum_{x \leq n \leq x+H} g(n) e(\alpha n)|\ dx. \ \ \ \ \ (5)$

In my previous paper with Matomaki and Radziwill, we could show that easier expression

$\displaystyle \frac{1}{X} \sup_\alpha \int_X^{2X} \frac{1}{H} |\sum_{x \leq n \leq x+H} g(n) e(\alpha n)|\ dx. \ \ \ \ \ (6)$

was small (for ${H}$ a slowly growing function of ${X}$ ) if ${g}$ was bounded and completely multiplicative, and one had a condition of the form

$\displaystyle \inf_{|t| \leq AX} \sum_{p \leq X} \frac{1 - \hbox{Re}( g(p) \overline{\chi(p)} p^{-it})}{p} \geq A \ \ \ \ \ (7)$

for some large ${A}$ . However, to obtain an analogous bound for (5) it now appears that one needs to strengthen the above condition to

$\displaystyle \inf_{|t| \leq AX^2} \sum_{p \leq X} \frac{1 - \hbox{Re}( g(p) \overline{\chi(p)} p^{-it})}{p} \geq A$

in order to address the counterexample in which ${g(n) = n^{it_0}}$ for some ${t_0}$ between ${X}$ and ${X^2}$ . This seems to suggest that proving (5) (which is closely related to the ${k=3}$ case of the Chowla conjecture) could in fact be rather difficult; the estimation of (6) relied primarily of prior work of Matomaki and Radziwill which used the hypothesis (7), but as this hypothesis is not sufficient to conclude (5), some additional input must also be used.

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Some recent papers

Fourier uniformity of bounded multiplicative functions in short intervals on average

The structure of correlations of multiplicative functions at almost all scales, with applications to the Chowla and Elliott conjectures

The logarithmically averaged and non-logarithmically averaged Chowla conjectures

Odd order cases of the logarithmically averaged Chowla conjecture

The structure of logarithmically averaged correlations of multiplicative functions, with applications to the Chowla and Elliott conjectures

Furstenberg limits of the Liouville function

Equivalence of the logarithmically averaged Chowla and Sarnak conjectures

A conjectural local Fourier-uniformity of the Liouville function

A small remark on the Elliott conjecture

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