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The Erdos discrepancy problem via the Elliott conjecture

11 September, 2015 in expository, math.NT, polymath | Tags: , , | by | 49 comments

The Chowla conjecture asserts that all non-trivial correlations of the Liouville function are asymptotically negligible; for instance, it asserts that

\displaystyle \sum_{n \leq X} \lambda(n) \lambda(n+h) = o(X)

as {X \rightarrow \infty} for any fixed natural number {h}. This conjecture remains open, though there are a number of partial results (e.g. these two previous results of Matomaki, Radziwill, and myself).

A natural generalisation of Chowla’s conjecture was proposed by Elliott. For simplicity we will only consider Elliott’s conjecture for the pair correlations

\displaystyle \sum_{n \leq X} g(n) \overline{g}(n+h).

For such correlations, the conjecture was that one had

\displaystyle \sum_{n \leq X} g(n) \overline{g}(n+h) = o(X) \ \ \ \ \ (1)

as {X \rightarrow \infty} for any natural number {h}, as long as {g} was a completely multiplicative function with magnitude bounded by {1}, and such that

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

for any Dirichlet character {\chi} and any real number {t}. In the language of “pretentious number theory”, as developed by Granville and Soundararajan, the hypothesis (2) asserts that the completely multiplicative function {g} does not “pretend” to be like the completely multiplicative function {n \mapsto \chi(n) n^{it}} for any character {\chi} and real number {t}. A condition of this form is necessary; for instance, if {g(n)} is precisely equal to {\chi(n) n^{it}} and {\chi} has period {q}, then {g(n) \overline{g}(n+q)} is equal to {1_{(n,q)=1} + o(1)} as {n \rightarrow \infty} and (1) clearly fails. The prime number theorem in arithmetic progressions implies that the Liouville function obeys (2), and so the Elliott conjecture contains the Chowla conjecture as a special case.

As it turns out, Elliott’s conjecture is false as stated, with the counterexample {g} having the property that {g} “pretends” locally to be the function {n \mapsto n^{it_j}} for {n} in various intervals {[1, X_j]}, where {X_j} and {t_j} go to infinity in a certain prescribed sense. See this paper of Matomaki, Radziwill, and myself for details. However, we view this as a technicality, and continue to believe that certain “repaired” versions of Elliott’s conjecture still hold. For instance, our counterexample does not apply when {g} is restricted to be real-valued rather than complex, and we believe that Elliott’s conjecture is valid in this setting. Returning to the complex-valued case, we still expect the asymptotic (1) provided that the condition (2) is replaced by the stronger condition

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

as {X \rightarrow +\infty} for all fixed Dirichlet characters {\chi}. In our paper we supported this claim by establishing a certain “averaged” version of this conjecture; see that paper for further details. (See also this recent paper of Frantzikinakis and Host which establishes a different averaged version of this conjecture.)

One can make a stronger “non-asymptotic” version of this corrected Elliott conjecture, in which the {X} parameter does not go to infinity, or equivalently that the function {g} is permitted to depend on {X}:

Conjecture 1 (Non-asymptotic Elliott conjecture) Let {\varepsilon > 0}, let {A \geq 1} be sufficiently large depending on {\varepsilon}, and let {X} be sufficiently large depending on {A,\varepsilon}. Suppose that {g} is a completely multiplicative function with magnitude bounded by {1}, such that

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

for all Dirichlet characters {\chi} of period at most {A}. Then one has

\displaystyle |\sum_{n \leq X} g(n) \overline{g(n+h)}| \leq \varepsilon X

for all natural numbers {1 \leq h \leq 1/\varepsilon}.

The {\varepsilon}-dependent factor {A} in the constraint {|t| \leq AX} is necessary, as can be seen by considering the completely multiplicative function {g(n) := n^{2iX}} (for instance). Again, the results in my previous paper with Matomaki and Radziwill can be viewed as establishing an averaged version of this conjecture.

Meanwhile, we have the following conjecture that is the focus of the Polymath5 project:

Conjecture 2 (Erdös discrepancy conjecture) For any function {f: {\bf N} \rightarrow \{-1,+1\}}, the discrepancy

\displaystyle \sup_{n,d \in {\bf N}} |\sum_{j=1}^n f(jd)|

is infinite.

It is instructive to compute some near-counterexamples to Conjecture 2 that illustrate the difficulty of the Erdös discrepancy problem. The first near-counterexample is that of a non-principal Dirichlet character {f(n) = \chi(n)} that takes values in {\{-1,0,+1\}} rather than {\{-1,+1\}}. For this function, one has from the complete multiplicativity of {\chi} that

\displaystyle |\sum_{j=1}^n f(jd)| = |\sum_{j=1}^n \chi(j) \chi(d)|

\displaystyle \leq |\sum_{j=1}^n \chi(j)|.

If {q} denotes the period of {\chi}, then {\chi} has mean zero on every interval of length {q}, and thus

\displaystyle |\sum_{j=1}^n f(jd)| \leq |\sum_{j=1}^n \chi(j)| \leq q.

Thus {\chi} has bounded discrepancy.

Of course, this is not a true counterexample to Conjecture 2 because {\chi} can take the value {0}. Let us now consider the following variant example, which is the simplest member of a family of examples studied by Borwein, Choi, and Coons. Let {\chi = \chi_3} be the non-principal Dirichlet character of period {3} (thus {\chi(n)} equals {+1} when {n=1 \hbox{ mod } 3}, {-1} when {n = 2 \hbox{ mod } 3}, and {0} when {n = 0 \hbox{ mod } 3}), and define the completely multiplicative function {f = \tilde \chi: {\bf N} \rightarrow \{-1,+1\}} by setting {\tilde \chi(p) := \chi(p)} when {p \neq 3} and {\tilde \chi(3) = +1}. This is about the simplest modification one can make to the previous near-counterexample to eliminate the zeroes. Now consider the sum

\displaystyle \sum_{j=1}^n \tilde \chi(j)

with {n := 1 + 3 + 3^2 + \dots + 3^k} for some large {k}. Writing {j = 3^a m} with {m} coprime to {3} and {a} at most {k}, we can write this sum as

\displaystyle \sum_{a=0}^k \sum_{1 \leq m \leq n/3^j} \tilde \chi(3^a m).

Now observe that {\tilde \chi(3^a m) = \tilde \chi(3)^a \tilde \chi(m) = \chi(m)}. The function {\chi} has mean zero on every interval of length three, and {\lfloor n/3^j\rfloor} is equal to {1} mod {3}, and thus

\displaystyle \sum_{1 \leq m \leq n/3^j} \tilde \chi(3^a m) = 1

for every {a=0,\dots,k}, and thus

\displaystyle \sum_{j=1}^n \tilde \chi(j) = k+1 \gg \log n.

Thus {\tilde \chi} also has unbounded discrepancy, but only barely so (it grows logarithmically in {n}). These examples suggest that the main “enemy” to proving Conjecture 2 comes from completely multiplicative functions {f} that somehow “pretend” to be like a Dirichlet character but do not vanish at the zeroes of that character. (Indeed, the special case of Conjecture 2 when {f} is completely multiplicative is already open, appears to be an important subcase.)

All of these conjectures remain open. However, I would like to record in this blog post the following striking connection, illustrating the power of the Elliott conjecture (particularly in its nonasymptotic formulation):

Theorem 3 (Elliott conjecture implies unbounded discrepancy) Conjecture 1 implies Conjecture 2.

The argument relies heavily on two observations that were previously made in connection with the Polymath5 project. The first is a Fourier-analytic reduction that replaces the Erdos Discrepancy Problem with an averaged version for completely multiplicative functions {g}. An application of Cauchy-Schwarz then shows that any counterexample to that version will violate the conclusion of Conjecture 1, so if one assumes that conjecture then {g} must pretend to be like a function of the form {n \mapsto \chi(n) n^{it}}. One then uses (a generalisation) of a second argument from Polymath5 to rule out this case, basically by reducing matters to a more complicated version of the Borwein-Choi-Coons analysis. Details are provided below the fold.

There is some hope that the Chowla and Elliott conjectures can be attacked, as the parity barrier which is so impervious to attack for the twin prime conjecture seems to be more permeable in this setting. (For instance, in my previous post I raised a possible approach, based on establishing expander properties of a certain random graph, which seems to get around the parity problem, in principle at least.)

(Update, Sep 25: fixed some treatment of error terms, following a suggestion of Andrew Granville.)

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