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[Note: the content of this post is standard number theoretic material that can be found in many textbooks (I am relying principally here on Iwaniec and Kowalski); I am not claiming any new progress on any version of the Riemann hypothesis here, but am simply arranging existing facts together.]

The Riemann hypothesis is arguably the most important and famous unsolved problem in number theory. It is usually phrased in terms of the Riemann zeta function ${\zeta}$, defined by

$\displaystyle \zeta(s) = \sum_{n=1}^\infty \frac{1}{n^s}$

for ${\hbox{Re}(s)>1}$ and extended meromorphically to other values of ${s}$, and asserts that the only zeroes of ${\zeta}$ in the critical strip ${\{ s: 0 \leq \hbox{Re}(s) \leq 1 \}}$ lie on the critical line ${\{ s: \hbox{Re}(s)=\frac{1}{2} \}}$.

One of the main reasons that the Riemann hypothesis is so important to number theory is that the zeroes of the zeta function in the critical strip control the distribution of the primes. To see the connection, let us perform the following formal manipulations (ignoring for now the important analytic issues of convergence of series, interchanging sums, branches of the logarithm, etc., in order to focus on the intuition). The starting point is the fundamental theorem of arithmetic, which asserts that every natural number ${n}$ has a unique factorisation ${n = p_1^{a_1} \ldots p_k^{a_k}}$ into primes. Taking logarithms, we obtain the identity

$\displaystyle \log n = \sum_{d|n} \Lambda(d) \ \ \ \ \ (1)$

for any natural number ${n}$, where ${\Lambda}$ is the von Mangoldt function, thus ${\Lambda(n) = \log p}$ when ${n}$ is a power of a prime ${p}$ and zero otherwise. If we then perform a “Dirichlet-Fourier transform” by viewing both sides of (1) as coefficients of a Dirichlet series, we conclude that

$\displaystyle \sum_{n=1}^\infty \frac{\log n}{n^s} = \sum_{n=1}^\infty \sum_{d|n} \frac{\Lambda(d)}{n^s},$

formally at least. Writing ${n=dm}$, the right-hand side factors as

$\displaystyle (\sum_{d=1}^\infty \frac{\Lambda(d)}{d^s}) (\sum_{m=1}^\infty \frac{1}{m^s}) = \zeta(s) \sum_{n=1}^\infty \frac{\Lambda(n)}{n^s}$

whereas the left-hand side is (formally, at least) equal to ${-\zeta'(s)}$. We conclude the identity

$\displaystyle \sum_{n=1}^\infty \frac{\Lambda(n)}{n^s} = -\frac{\zeta'(s)}{\zeta(s)},$

(formally, at least). If we integrate this, we are formally led to the identity

$\displaystyle \sum_{n=1}^\infty \frac{\Lambda(n)}{\log n} n^{-s} = \log \zeta(s)$

or equivalently to the exponential identity

$\displaystyle \zeta(s) = \exp( \sum_{n=1}^\infty \frac{\Lambda(n)}{\log n} n^{-s} ) \ \ \ \ \ (2)$

which allows one to reconstruct the Riemann zeta function from the von Mangoldt function. (It is instructive exercise in enumerative combinatorics to try to prove this identity directly, at the level of formal Dirichlet series, using the fundamental theorem of arithmetic of course.) Now, as ${\zeta}$ has a simple pole at ${s=1}$ and zeroes at various places ${s=\rho}$ on the critical strip, we expect a Weierstrass factorisation which formally (ignoring normalisation issues) takes the form

$\displaystyle \zeta(s) = \frac{1}{s-1} \times \prod_\rho (s-\rho) \times \ldots$

(where we will be intentionally vague about what is hiding in the ${\ldots}$ terms) and so we expect an expansion of the form

$\displaystyle \sum_{n=1}^\infty \frac{\Lambda(n)}{\log n} n^{-s} = - \log(s-1) + \sum_\rho \log(s-\rho) + \ldots. \ \ \ \ \ (3)$

Note that

$\displaystyle \frac{1}{s-\rho} = \int_1^\infty t^{\rho-s} \frac{dt}{t}$

and hence on integrating in ${s}$ we formally have

$\displaystyle \log(s-\rho) = -\int_1^\infty t^{\rho-s-1} \frac{dt}{\log t}$

and thus we have the heuristic approximation

$\displaystyle \log(s-\rho) \approx - \sum_{n=1}^\infty \frac{n^{\rho-s-1}}{\log n}.$

Comparing this with (3), we are led to a heuristic form of the explicit formula

$\displaystyle \Lambda(n) \approx 1 - \sum_\rho n^{\rho-1}. \ \ \ \ \ (4)$

When trying to make this heuristic rigorous, it turns out (due to the rough nature of both sides of (4)) that one has to interpret the explicit formula in some suitably weak sense, for instance by testing (4) against the indicator function ${1_{[0,x]}(n)}$ to obtain the formula

$\displaystyle \sum_{n \leq x} \Lambda(n) \approx x - \sum_\rho \frac{x^{\rho}}{\rho} \ \ \ \ \ (5)$

which can in fact be made into a rigorous statement after some truncation (the von Mangoldt explicit formula). From this formula we now see how helpful the Riemann hypothesis will be to control the distribution of the primes; indeed, if the Riemann hypothesis holds, so that ${\hbox{Re}(\rho) = 1/2}$ for all zeroes ${\rho}$, it is not difficult to use (a suitably rigorous version of) the explicit formula to conclude that

$\displaystyle \sum_{n \leq x} \Lambda(n) = x + O( x^{1/2} \log^2 x ) \ \ \ \ \ (6)$

as ${x \rightarrow \infty}$, giving a near-optimal “square root cancellation” for the sum ${\sum_{n \leq x} \Lambda(n)-1}$. Conversely, if one can somehow establish a bound of the form

$\displaystyle \sum_{n \leq x} \Lambda(n) = x + O( x^{1/2+\epsilon} )$

for any fixed ${\epsilon}$, then the explicit formula can be used to then deduce that all zeroes ${\rho}$ of ${\zeta}$ have real part at most ${1/2+\epsilon}$, which leads to the following remarkable amplification phenomenon (analogous, as we will see later, to the tensor power trick): any bound of the form

$\displaystyle \sum_{n \leq x} \Lambda(n) = x + O( x^{1/2+o(1)} )$

can be automatically amplified to the stronger bound

$\displaystyle \sum_{n \leq x} \Lambda(n) = x + O( x^{1/2} \log^2 x )$

with both bounds being equivalent to the Riemann hypothesis. Of course, the Riemann hypothesis for the Riemann zeta function remains open; but partial progress on this hypothesis (in the form of zero-free regions for the zeta function) leads to partial versions of the asymptotic (6). For instance, it is known that there are no zeroes of the zeta function on the line ${\hbox{Re}(s)=1}$, and this can be shown by some analysis (either complex analysis or Fourier analysis) to be equivalent to the prime number theorem

$\displaystyle \sum_{n \leq x} \Lambda(n) =x + o(x);$

see e.g. this previous blog post for more discussion.

The main engine powering the above observations was the fundamental theorem of arithmetic, and so one can expect to establish similar assertions in other contexts where some version of the fundamental theorem of arithmetic is available. One of the simplest such variants is to continue working on the natural numbers, but “twist” them by a Dirichlet character ${\chi: {\bf Z} \rightarrow {\bf R}}$. The analogue of the Riemann zeta function is then the (1), which encoded the fundamental theorem of arithmetic, can be twisted by ${\chi}$ to obtain

$\displaystyle \chi(n) \log n = \sum_{d|n} \chi(d) \Lambda(d) \chi(\frac{n}{d}) \ \ \ \ \ (7)$

and essentially the same manipulations as before eventually lead to the exponential identity

$\displaystyle L(s,\chi) = \exp( \sum_{n=1}^\infty \frac{\chi(n) \Lambda(n)}{\log n} n^{-s} ). \ \ \ \ \ (8)$

which is a twisted version of (2), as well as twisted explicit formula, which heuristically takes the form

$\displaystyle \chi(n) \Lambda(n) \approx - \sum_\rho n^{\rho-1} \ \ \ \ \ (9)$

for non-principal ${\chi}$, where ${\rho}$ now ranges over the zeroes of ${L(s,\chi)}$ in the critical strip, rather than the zeroes of ${\zeta(s)}$; a more accurate formulation, following (5), would be

$\displaystyle \sum_{n \leq x} \chi(n) \Lambda(n) \approx - \sum_\rho \frac{x^{\rho}}{\rho}. \ \ \ \ \ (10)$

(See e.g. Davenport’s book for a more rigorous discussion which emphasises the analogy between the Riemann zeta function and the Dirichlet ${L}$-function.) If we assume the generalised Riemann hypothesis, which asserts that all zeroes of ${L(s,\chi)}$ in the critical strip also lie on the critical line, then we obtain the bound

$\displaystyle \sum_{n \leq x} \chi(n) \Lambda(n) = O( x^{1/2} \log(x) \log(xq) )$

for any non-principal Dirichlet character ${\chi}$, again demonstrating a near-optimal square root cancellation for this sum. Again, we have the amplification property that the above bound is implied by the apparently weaker bound

$\displaystyle \sum_{n \leq x} \chi(n) \Lambda(n) = O( x^{1/2+o(1)} )$

(where ${o(1)}$ denotes a quantity that goes to zero as ${x \rightarrow \infty}$ for any fixed ${q}$). Next, one can consider other number systems than the natural numbers ${{\bf N}}$ and integers ${{\bf Z}}$. For instance, one can replace the integers ${{\bf Z}}$ with rings ${{\mathcal O}_K}$ of integers in other number fields ${K}$ (i.e. finite extensions of ${{\bf Q}}$), such as the quadratic extensions ${K = {\bf Q}[\sqrt{D}]}$ of the rationals for various square-free integers ${D}$, in which case the ring of integers would be the ring of quadratic integers ${{\mathcal O}_K = {\bf Z}[\omega]}$ for a suitable generator ${\omega}$ (it turns out that one can take ${\omega = \sqrt{D}}$ if ${D=2,3\hbox{ mod } 4}$, and ${\omega = \frac{1+\sqrt{D}}{2}}$ if ${D=1 \hbox{ mod } 4}$). Here, it is not immediately obvious what the analogue of the natural numbers ${{\bf N}}$ is in this setting, since rings such as ${{\bf Z}[\omega]}$ do not come with a natural ordering. However, we can adopt an algebraic viewpoint to see the correct generalisation, observing that every natural number ${n}$ generates a principal ideal ${(n) = \{ an: a \in {\bf Z} \}}$ in the integers, and conversely every non-trivial ideal ${{\mathfrak n}}$ in the integers is associated to precisely one natural number ${n}$ in this fashion, namely the norm ${N({\mathfrak n}) := |{\bf Z} / {\mathfrak n}|}$ of that ideal. So one can identify the natural numbers with the ideals of ${{\bf Z}}$. Furthermore, with this identification, the prime numbers correspond to the prime ideals, since if ${p}$ is prime, and ${a,b}$ are integers, then ${ab \in (p)}$ if and only if one of ${a \in (p)}$ or ${b \in (p)}$ is true. Finally, even in number systems (such as ${{\bf Z}[\sqrt{-5}]}$) in which the classical version of the fundamental theorem of arithmetic fail (e.g. ${6 = 2 \times 3 = (1-\sqrt{-5})(1+\sqrt{-5})}$), we have the fundamental theorem of arithmetic for ideals: every ideal ${\mathfrak{n}}$ in a Dedekind domain (which includes the ring ${{\mathcal O}_K}$ of integers in a number field as a key example) is uniquely representable (up to permutation) as the product of a finite number of prime ideals ${\mathfrak{p}}$ (although these ideals might not necessarily be principal). For instance, in ${{\bf Z}[\sqrt{-5}]}$, the principal ideal ${(6)}$ factors as the product of four prime (but non-principal) ideals ${(2, 1+\sqrt{-5})}$, ${(2, 1-\sqrt{-5})}$, ${(3, 1+\sqrt{-5})}$, ${(3, 1-\sqrt{-5})}$. (Note that the first two ideals ${(2,1+\sqrt{5}), (2,1-\sqrt{5})}$ are actually equal to each other.) Because we still have the fundamental theorem of arithmetic, we can develop analogues of the previous observations relating the Riemann hypothesis to the distribution of primes. The analogue of the Riemann hypothesis is now the Dedekind zeta function

$\displaystyle \zeta_K(s) := \sum_{{\mathfrak n}} \frac{1}{N({\mathfrak n})^s}$

where the summation is over all non-trivial ideals in ${{\mathcal O}_K}$. One can also define a von Mangoldt function ${\Lambda_K({\mathfrak n})}$, defined as ${\log N( {\mathfrak p})}$ when ${{\mathfrak n}}$ is a power of a prime ideal ${{\mathfrak p}}$, and zero otherwise; then the fundamental theorem of arithmetic for ideals can be encoded in an analogue of (1) (or (7)),

$\displaystyle \log N({\mathfrak n}) = \sum_{{\mathfrak d}|{\mathfrak n}} \Lambda_K({\mathfrak d}) \ \ \ \ \ (11)$

which leads as before to an exponential identity

$\displaystyle \zeta_K(s) = \exp( \sum_{{\mathfrak n}} \frac{\Lambda_K({\mathfrak n})}{\log N({\mathfrak n})} N({\mathfrak n})^{-s} ) \ \ \ \ \ (12)$

and an explicit formula of the heuristic form

$\displaystyle \Lambda({\mathfrak n}) \approx 1 - \sum_\rho N({\mathfrak n})^{\rho-1}$

or, a little more accurately,

$\displaystyle \sum_{N({\mathfrak n}) \leq x} \Lambda({\mathfrak n}) \approx x - \sum_\rho \frac{x^{\rho}}{\rho} \ \ \ \ \ (13)$

in analogy with (5) or (10). Again, a suitable Riemann hypothesis for the Dedekind zeta function leads to good asymptotics for the distribution of prime ideals, giving a bound of the form

$\displaystyle \sum_{N({\mathfrak n}) \leq x} \Lambda({\mathfrak n}) = x + O( \sqrt{x} \log(x) (d+\log(Dx)) )$

where ${D}$ is the conductor of ${K}$ (which, in the case of number fields, is the absolute value of the discriminant of ${K}$) and ${d = \hbox{dim}_{\bf Q}(K)}$ is the degree of the extension of ${K}$ over ${{\bf Q}}$. As before, we have the amplification phenomenon that the above near-optimal square root cancellation bound is implied by the weaker bound

$\displaystyle \sum_{N({\mathfrak n}) \leq x} \Lambda({\mathfrak n}) = x + O( x^{1/2+o(1)} )$

where ${o(1)}$ denotes a quantity that goes to zero as ${x \rightarrow \infty}$ (holding ${K}$ fixed). See e.g. Chapter 5 of Iwaniec-Kowalski for details.

As was the case with the Dirichlet ${L}$-functions, one can twist the Dedekind zeta function example by characters, in this case the Hecke characters; we will not do this here, but see e.g. Section 3 of Iwaniec-Kowalski for details.

Very analogous considerations hold if we move from number fields to function fields. The simplest case is the function field associated to the affine line ${{\mathbb A}^1}$ and a finite field ${{\mathbb F} = {\mathbb F}_q}$ of some order ${q}$. The polynomial functions on the affine line ${{\mathbb A}^1/{\mathbb F}}$ are just the usual polynomial ring ${{\mathbb F}[t]}$, which then play the role of the integers ${{\bf Z}}$ (or ${{\mathcal O}_K}$) in previous examples. This ring happens to be a unique factorisation domain, so the situation is closely analogous to the classical setting of the Riemann zeta function. The analogue of the natural numbers are the monic polynomials (since every non-trivial principal ideal is generated by precisely one monic polynomial), and the analogue of the prime numbers are the irreducible monic polynomials. The norm ${N(f)}$ of a polynomial is the order of ${{\mathbb F}[t] / (f)}$, which can be computed explicitly as

$\displaystyle N(f) = q^{\hbox{deg}(f)}.$

Because of this, we will normalise things slightly differently here and use ${\hbox{deg}(f)}$ in place of ${\log N(f)}$ in what follows. The (local) zeta function ${\zeta_{{\mathbb A}^1/{\mathbb F}}(s)}$ is then defined as

$\displaystyle \zeta_{{\mathbb A}^1/{\mathbb F}}(s) = \sum_f \frac{1}{N(f)^s}$

where ${f}$ ranges over monic polynomials, and the von Mangoldt function ${\Lambda_{{\mathbb A}^1/{\mathbb F}}(f)}$ is defined to equal ${\hbox{deg}(g)}$ when ${f}$ is a power of a monic irreducible polynomial ${g}$, and zero otherwise. Note that because ${N(f)}$ is always a power of ${q}$, the zeta function here is in fact periodic with period ${2\pi i / \log q}$. Because of this, it is customary to make a change of variables ${T := q^{-s}}$, so that

$\displaystyle \zeta_{{\mathbb A}^1/{\mathbb F}}(s) = Z( {\mathbb A}^1/{\mathbb F}, T )$

and ${Z}$ is the renormalised zeta function

$\displaystyle Z( {\mathbb A}^1/{\mathbb F}, T ) = \sum_f T^{\hbox{deg}(f)}. \ \ \ \ \ (14)$

We have the analogue of (1) (or (7) or (11)):

$\displaystyle \hbox{deg}(f) = \sum_{g|f} \Lambda_{{\mathbb A}^1/{\mathbb F}}(g), \ \ \ \ \ (15)$

which leads as before to an exponential identity

$\displaystyle Z( {\mathbb A}^1/{\mathbb F}, T ) = \exp( \sum_f \frac{\Lambda_{{\mathbb A}^1/{\mathbb F}}(f)}{\hbox{deg}(f)} T^{\hbox{deg}(f)} ) \ \ \ \ \ (16)$

analogous to (2), (8), or (12). It also leads to the explicit formula

$\displaystyle \Lambda_{{\mathbb A}^1/{\mathbb F}}(f) \approx 1 - \sum_\rho N(f)^{\rho-1}$

where ${\rho}$ are the zeroes of the original zeta function ${\zeta_{{\mathbb A}^1/{\mathbb F}}(s)}$ (counting each residue class of the period ${2\pi i/\log q}$ just once), or equivalently

$\displaystyle \Lambda_{{\mathbb A}^1/{\mathbb F}}(f) \approx 1 - q^{-\hbox{deg}(f)} \sum_\alpha \alpha^{\hbox{deg}(f)},$

where ${\alpha}$ are the reciprocals of the roots of the normalised zeta function ${Z( {\mathbb A}^1/{\mathbb F}, T )}$ (or to put it another way, ${1-\alpha T}$ are the factors of this zeta function). Again, to make proper sense of this heuristic we need to sum, obtaining

$\displaystyle \sum_{\hbox{deg}(f) = n} \Lambda_{{\mathbb A}^1/{\mathbb F}}(f) \approx q^n - \sum_\alpha \alpha^n.$

As it turns out, in the function field setting, the zeta functions are always rational (this is part of the Weil conjectures), and the above heuristic formula is basically exact up to a constant factor, thus

$\displaystyle \sum_{\hbox{deg}(f) = n} \Lambda_{{\mathbb A}^1/{\mathbb F}}(f) = q^n - \sum_\alpha \alpha^n + c \ \ \ \ \ (17)$

for an explicit integer ${c}$ (independent of ${n}$) arising from any potential pole of ${Z}$ at ${T=1}$. In the case of the affine line ${{\mathbb A}^1}$, the situation is particularly simple, because the zeta function ${Z( {\mathbb A}^1/{\mathbb F}, T)}$ is easy to compute. Indeed, since there are exactly ${q^n}$ monic polynomials of a given degree ${n}$, we see from (14) that

$\displaystyle Z( {\mathbb A}^1/{\mathbb F}, T ) = \sum_{n=0}^\infty q^n T^n = \frac{1}{1-qT}$

so in fact there are no zeroes whatsoever, and no pole at ${T=1}$ either, so we have an exact prime number theorem for this function field:

$\displaystyle \sum_{\hbox{deg}(f) = n} \Lambda_{{\mathbb A}^1/{\mathbb F}}(f) = q^n \ \ \ \ \ (18)$

Among other things, this tells us that the number of irreducible monic polynomials of degree ${n}$ is ${q^n/n + O(q^{n/2})}$.

We can transition from an algebraic perspective to a geometric one, by viewing a given monic polynomial ${f \in {\mathbb F}[t]}$ through its roots, which are a finite set of points in the algebraic closure ${\overline{{\mathbb F}}}$ of the finite field ${{\mathbb F}}$ (or more suggestively, as points on the affine line ${{\mathbb A}^1( \overline{{\mathbb F}} )}$). The number of such points (counting multiplicity) is the degree of ${f}$, and from the factor theorem, the set of points determines the monic polynomial ${f}$ (or, if one removes the monic hypothesis, it determines the polynomial ${f}$ projectively). These points have an action of the Galois group ${\hbox{Gal}( \overline{{\mathbb F}} / {\mathbb F} )}$. It is a classical fact that this Galois group is in fact a cyclic group generated by a single element, the (geometric) Frobenius map ${\hbox{Frob}: x \mapsto x^q}$, which fixes the elements of the original finite field ${{\mathbb F}}$ but permutes the other elements of ${\overline{{\mathbb F}}}$. Thus the roots of a given polynomial ${f}$ split into orbits of the Frobenius map. One can check that the roots consist of a single such orbit (counting multiplicity) if and only if ${f}$ is irreducible; thus the fundamental theorem of arithmetic can be viewed geometrically as as the orbit decomposition of any Frobenius-invariant finite set of points in the affine line.

Now consider the degree ${n}$ finite field extension ${{\mathbb F}_n}$ of ${{\mathbb F}}$ (it is a classical fact that there is exactly one such extension up to isomorphism for each ${n}$); this is a subfield of ${\overline{{\mathbb F}}}$ of order ${q^n}$. (Here we are performing a standard abuse of notation by overloading the subscripts in the ${{\mathbb F}}$ notation; thus ${{\mathbb F}_q}$ denotes the field of order ${q}$, while ${{\mathbb F}_n}$ denotes the extension of ${{\mathbb F} = {\mathbb F}_q}$ of order ${n}$, so that we in fact have ${{\mathbb F}_n = {\mathbb F}_{q^n}}$ if we use one subscript convention on the left-hand side and the other subscript convention on the right-hand side. We hope this overloading will not cause confusion.) Each point ${x}$ in this extension (or, more suggestively, the affine line ${{\mathbb A}^1({\mathbb F}_n)}$ over this extension) has a minimal polynomial – an irreducible monic polynomial whose roots consist the Frobenius orbit of ${x}$. Since the Frobenius action is periodic of period ${n}$ on ${{\mathbb F}_n}$, the degree of this minimal polynomial must divide ${n}$. Conversely, every monic irreducible polynomial of degree ${d}$ dividing ${n}$ produces ${d}$ distinct zeroes that lie in ${{\mathbb F}_d}$ (here we use the classical fact that finite fields are perfect) and hence in ${{\mathbb F}_n}$. We have thus partitioned ${{\mathbb A}^1({\mathbb F}_n)}$ into Frobenius orbits (also known as closed points), with each monic irreducible polynomial ${f}$ of degree ${d}$ dividing ${n}$ contributing an orbit of size ${d = \hbox{deg}(f) = \Lambda(f^{n/d})}$. From this we conclude a geometric interpretation of the left-hand side of (18):

$\displaystyle \sum_{\hbox{deg}(f) = n} \Lambda_{{\mathbb A}^1/{\mathbb F}}(f) = \sum_{x \in {\mathbb A}^1({\mathbb F}_n)} 1. \ \ \ \ \ (19)$

The identity (18) thus is equivalent to the thoroughly boring fact that the number of ${{\mathbb F}_n}$-points on the affine line ${{\mathbb A}^1}$ is equal to ${q^n}$. However, things become much more interesting if one then replaces the affine line ${{\mathbb A}^1}$ by a more general (geometrically) irreducible curve ${C}$ defined over ${{\mathbb F}}$; for instance one could take ${C}$ to be an ellpitic curve

$\displaystyle E = \{ (x,y): y^2 = x^3 + ax + b \} \ \ \ \ \ (20)$

for some suitable ${a,b \in {\mathbb F}}$, although the discussion here applies to more general curves as well (though to avoid some minor technicalities, we will assume that the curve is projective with a finite number of ${{\mathbb F}}$-rational points removed). The analogue of ${{\mathbb F}[t]}$ is then the coordinate ring of ${C}$ (for instance, in the case of the elliptic curve (20) it would be ${{\mathbb F}[x,y] / (y^2-x^3-ax-b)}$), with polynomials in this ring producing a set of roots in the curve ${C( \overline{\mathbb F})}$ that is again invariant with respect to the Frobenius action (acting on the ${x}$ and ${y}$ coordinates separately). In general, we do not expect unique factorisation in this coordinate ring (this is basically because Bezout’s theorem suggests that the zero set of a polynomial on ${C}$ will almost never consist of a single (closed) point). Of course, we can use the algebraic formalism of ideals to get around this, setting up a zeta function

$\displaystyle \zeta_{C/{\mathbb F}}(s) = \sum_{{\mathfrak f}} \frac{1}{N({\mathfrak f})^s}$

and a von Mangoldt function ${\Lambda_{C/{\mathbb F}}({\mathfrak f})}$ as before, where ${{\mathfrak f}}$ would now run over the non-trivial ideals of the coordinate ring. However, it is more instructive to use the geometric viewpoint, using the ideal-variety dictionary from algebraic geometry to convert algebraic objects involving ideals into geometric objects involving varieties. In this dictionary, a non-trivial ideal would correspond to a proper subvariety (or more precisely, a subscheme, but let us ignore the distinction between varieties and schemes here) of the curve ${C}$; as the curve is irreducible and one-dimensional, this subvariety must be zero-dimensional and is thus a (multi-)set of points ${\{x_1,\ldots,x_k\}}$ in ${C}$, or equivalently an effective divisor ${[x_1] + \ldots + [x_k]}$ of ${C}$; this generalises the concept of the set of roots of a polynomial (which corresponds to the case of a principal ideal). Furthermore, this divisor has to be rational in the sense that it is Frobenius-invariant. The prime ideals correspond to those divisors (or sets of points) which are irreducible, that is to say the individual Frobenius orbits, also known as closed points of ${C}$. With this dictionary, the zeta function becomes

$\displaystyle \zeta_{C/{\mathbb F}}(s) = \sum_{D \geq 0} \frac{1}{q^{\hbox{deg}(D)}}$

where the sum is over effective rational divisors ${D}$ of ${C}$ (with ${k}$ being the degree of an effective divisor ${[x_1] + \ldots + [x_k]}$), or equivalently

$\displaystyle Z( C/{\mathbb F}, T ) = \sum_{D \geq 0} T^{\hbox{deg}(D)}.$

The analogue of (19), which gives a geometric interpretation to sums of the von Mangoldt function, becomes

$\displaystyle \sum_{N({\mathfrak f}) = q^n} \Lambda_{C/{\mathbb F}}({\mathfrak f}) = \sum_{x \in C({\mathbb F}_n)} 1$

$\displaystyle = |C({\mathbb F}_n)|,$

thus this sum is simply counting the number of ${{\mathbb F}_n}$-points of ${C}$. The analogue of the exponential identity (16) (or (2), (8), or (12)) is then

$\displaystyle Z( C/{\mathbb F}, T ) = \exp( \sum_{n \geq 1} \frac{|C({\mathbb F}_n)|}{n} T^n ) \ \ \ \ \ (21)$

and the analogue of the explicit formula (17) (or (5), (10) or (13)) is

$\displaystyle |C({\mathbb F}_n)| = q^n - \sum_\alpha \alpha^n + c \ \ \ \ \ (22)$

where ${\alpha}$ runs over the (reciprocal) zeroes of ${Z( C/{\mathbb F}, T )}$ (counting multiplicity), and ${c}$ is an integer independent of ${n}$. (As it turns out, ${c}$ equals ${1}$ when ${C}$ is a projective curve, and more generally equals ${1-k}$ when ${C}$ is a projective curve with ${k}$ rational points deleted.)

To evaluate ${Z(C/{\mathbb F},T)}$, one needs to count the number of effective divisors of a given degree on the curve ${C}$. Fortunately, there is a tool that is particularly well-designed for this task, namely the Riemann-Roch theorem. By using this theorem, one can show (when ${C}$ is projective) that ${Z(C/{\mathbb F},T)}$ is in fact a rational function, with a finite number of zeroes, and a simple pole at both ${1}$ and ${1/q}$, with similar results when one deletes some rational points from ${C}$; see e.g. Chapter 11 of Iwaniec-Kowalski for details. Thus the sum in (22) is finite. For instance, for the affine elliptic curve (20) (which is a projective curve with one point removed), it turns out that we have

$\displaystyle |E({\mathbb F}_n)| = q^n - \alpha^n - \beta^n$

for two complex numbers ${\alpha,\beta}$ depending on ${E}$ and ${q}$.

The Riemann hypothesis for (untwisted) curves – which is the deepest and most difficult aspect of the Weil conjectures for these curves – asserts that the zeroes of ${\zeta_{C/{\mathbb F}}}$ lie on the critical line, or equivalently that all the roots ${\alpha}$ in (22) have modulus ${\sqrt{q}}$, so that (22) then gives the asymptotic

$\displaystyle |C({\mathbb F}_n)| = q^n + O( q^{n/2} ) \ \ \ \ \ (23)$

where the implied constant depends only on the genus of ${C}$ (and on the number of points removed from ${C}$). For instance, for elliptic curves we have the Hasse bound

$\displaystyle |E({\mathbb F}_n) - q^n| \leq 2 \sqrt{q}.$

As before, we have an important amplification phenomenon: if we can establish a weaker estimate, e.g.

$\displaystyle |C({\mathbb F}_n)| = q^n + O( q^{n/2 + O(1)} ), \ \ \ \ \ (24)$

then we can automatically deduce the stronger bound (23). This amplification is not a mere curiosity; most of the proofs of the Riemann hypothesis for curves proceed via this fact. For instance, by using the elementary method of Stepanov to bound points in curves (discussed for instance in this previous post), one can establish the preliminary bound (24) for large ${n}$, which then amplifies to the optimal bound (23) for all ${n}$ (and in particular for ${n=1}$). Again, see Chapter 11 of Iwaniec-Kowalski for details. The ability to convert a bound with ${q}$-dependent losses over the optimal bound (such as (24)) into an essentially optimal bound with no ${q}$-dependent losses (such as (23)) is important in analytic number theory, since in many applications (e.g. in those arising from sieve theory) one wishes to sum over large ranges of ${q}$.

Much as the Riemann zeta function can be twisted by a Dirichlet character to form a Dirichlet ${L}$-function, one can twist the zeta function on curves by various additive and multiplicative characters. For instance, suppose one has an affine plane curve ${C \subset {\mathbb A}^2}$ and an additive character ${\psi: {\mathbb F}^2 \rightarrow {\bf C}^\times}$, thus ${\psi(x+y) = \psi(x) \psi(y)}$ for all ${x,y \in {\mathbb F}^2}$. Given a rational effective divisor ${D = [x_1] + \ldots + [x_k]}$, the sum ${x_1+\ldots+x_k}$ is Frobenius-invariant and thus lies in ${{\mathbb F}^2}$. By abuse of notation, we may thus define ${\psi}$ on such divisors by

$\displaystyle \psi( [x_1] + \ldots + [x_k] ) := \psi( x_1 + \ldots + x_k )$

and observe that ${\psi}$ is multiplicative in the sense that ${\psi(D_1 + D_2) = \psi(D_1) \psi(D_2)}$ for rational effective divisors ${D_1,D_2}$. One can then define ${\psi({\mathfrak f})}$ for any non-trivial ideal ${{\mathfrak f}}$ by replacing that ideal with the associated rational effective divisor; for instance, if ${f}$ is a polynomial in the coefficient ring of ${C}$, with zeroes at ${x_1,\ldots,x_k \in C}$, then ${\psi((f))}$ is ${\psi( x_1+\ldots+x_k )}$. Again, we have the multiplicativity property ${\psi({\mathfrak f} {\mathfrak g}) = \psi({\mathfrak f}) \psi({\mathfrak g})}$. If we then form the twisted normalised zeta function

$\displaystyle Z( C/{\mathbb F}, \psi, T ) = \sum_{D \geq 0} \psi(D) T^{\hbox{deg}(D)}$

then by twisting the previous analysis, we eventually arrive at the exponential identity

$\displaystyle Z( C/{\mathbb F}, \psi, T ) = \exp( \sum_{n \geq 1} \frac{S_n(C/{\mathbb F}, \psi)}{n} T^n ) \ \ \ \ \ (25)$

in analogy with (21) (or (2), (8), (12), or (16)), where the companion sums ${S_n(C/{\mathbb F}, \psi)}$ are defined by

$\displaystyle S_n(C/{\mathbb F},\psi) = \sum_{x \in C({\mathbb F}^n)} \psi( \hbox{Tr}_{{\mathbb F}_n/{\mathbb F}}(x) )$

where the trace ${\hbox{Tr}_{{\mathbb F}_n/{\mathbb F}}(x)}$ of an element ${x}$ in the plane ${{\mathbb A}^2( {\mathbb F}_n )}$ is defined by the formula

$\displaystyle \hbox{Tr}_{{\mathbb F}_n/{\mathbb F}}(x) = x + \hbox{Frob}(x) + \ldots + \hbox{Frob}^{n-1}(x).$

In particular, ${S_1}$ is the exponential sum

$\displaystyle S_1(C/{\mathbb F},\psi) = \sum_{x \in C({\mathbb F})} \psi(x)$

which is an important type of sum in analytic number theory, containing for instance the Kloosterman sum

$\displaystyle K(a,b;p) := \sum_{x \in {\mathbb F}_p^\times} e_p( ax + \frac{b}{x})$

as a special case, where ${a,b \in {\mathbb F}_p^\times}$. (NOTE: the sign conventions for the companion sum ${S_n}$ are not consistent across the literature, sometimes it is ${-S_n}$ which is referred to as the companion sum.)

If ${\psi}$ is non-principal (and ${C}$ is non-linear), one can show (by a suitably twisted version of the Riemann-Roch theorem) that ${Z}$ is a rational function of ${T}$, with no pole at ${T=q^{-1}}$, and one then gets an explicit formula of the form

$\displaystyle S_n(C/{\mathbb F},\psi) = -\sum_\alpha \alpha^n + c \ \ \ \ \ (26)$

for the companion sums, where ${\alpha}$ are the reciprocals of the zeroes of ${S}$, in analogy to (22) (or (5), (10), (13), or (17)). For instance, in the case of Kloosterman sums, there is an identity of the form

$\displaystyle \sum_{x \in {\mathbb F}_{p^n}^\times} e_p( a \hbox{Tr}(x) + \frac{b}{\hbox{Tr}(x)}) = -\alpha^n - \beta^n \ \ \ \ \ (27)$

for all ${n}$ and some complex numbers ${\alpha,\beta}$ depending on ${a,b,p}$, where we have abbreviated ${\hbox{Tr}_{{\mathbb F}_{p^n}/{\mathbb F}_p}}$ as ${\hbox{Tr}}$. As before, the Riemann hypothesis for ${Z}$ then gives a square root cancellation bound of the form

$\displaystyle S_n(C/{\mathbb F},\psi) = O( q^{n/2} ) \ \ \ \ \ (28)$

for the companion sums (and in particular gives the very explicit Weil bound ${|K(a,b;p)| \leq 2\sqrt{p}}$ for the Kloosterman sum), but again there is the amplification phenomenon that this sort of bound can be deduced from the apparently weaker bound

$\displaystyle S_n(C/{\mathbb F},\psi) = O( q^{n/2 + O(1)} ).$

As before, most of the known proofs of the Riemann hypothesis for these twisted zeta functions proceed by first establishing this weaker bound (e.g. one could again use Stepanov’s method here for this goal) and then amplifying to the full bound (28); see Chapter 11 of Iwaniec-Kowalski for further details.

One can also twist the zeta function on a curve by a multiplicative character ${\chi: {\mathbb F}^\times \rightarrow {\bf C}^\times}$ by similar arguments, except that instead of forming the sum ${x_1+\ldots+x_k}$ of all the components of an effective divisor ${[x_1]+\ldots+[x_k]}$, one takes the product ${x_1 \ldots x_k}$ instead, and similarly one replaces the trace

$\displaystyle \hbox{Tr}_{{\mathbb F}_n/{\mathbb F}}(x) = x + \hbox{Frob}(x) + \ldots + \hbox{Frob}^{n-1}(x)$

by the norm

$\displaystyle \hbox{Norm}_{{\mathbb F}_n/{\mathbb F}}(x) = x \cdot \hbox{Frob}(x) \cdot \ldots \cdot \hbox{Frob}^{n-1}(x).$

Again, see Chapter 11 of Iwaniec-Kowalski for details.

Deligne famously extended the above theory to higher-dimensional varieties than curves, and also to the closely related context of ${\ell}$-adic sheaves on curves, giving rise to two separate proofs of the Weil conjectures in full generality. (Very roughly speaking, the former context can be obtained from the latter context by a sort of Fubini theorem type argument that expresses sums on higher-dimensional varieties as iterated sums on curves of various expressions related to ${\ell}$-adic sheaves.) In this higher-dimensional setting, the zeta function formalism is still present, but is much more difficult to use, in large part due to the much less tractable nature of divisors in higher dimensions (they are now combinations of codimension one subvarieties or subschemes, rather than combinations of points). To get around this difficulty, one has to change perspective yet again, from an algebraic or geometric perspective to an ${\ell}$-adic cohomological perspective. (I could imagine that once one is sufficiently expert in the subject, all these perspectives merge back together into a unified viewpoint, but I am certainly not yet at that stage of understanding.) In particular, the zeta function, while still present, plays a significantly less prominent role in the analysis (at least if one is willing to take Deligne’s theorems as a black box); the explicit formula is now obtained via a different route, namely the Grothendieck-Lefschetz fixed point formula. I have written some notes on this material below the fold (based in part on some lectures of Philippe Michel, as well as the text of Iwaniec-Kowalski and also this book of Katz), but I should caution that my understanding here is still rather sketchy and possibly inaccurate in places.

As in previous posts, we use the following asymptotic notation: ${x}$ is a parameter going off to infinity, and all quantities may depend on ${x}$ unless explicitly declared to be “fixed”. The asymptotic notation ${O(), o(), \ll}$ is then defined relative to this parameter. A quantity ${q}$ is said to be of polynomial size if one has ${q = O(x^{O(1)})}$, and said to be bounded if ${q=O(1)}$. Another convenient notation: we write ${X \lessapprox Y}$ for ${X \ll x^{o(1)} Y}$. Thus for instance the divisor bound asserts that if ${q}$ has polynomial size, then the number of divisors of ${q}$ is ${\lessapprox 1}$.

This post is intended to highlight a phenomenon unearthed in the ongoing polymath8 project (and is in fact a key component of Zhang’s proof that there are bounded gaps between primes infinitely often), namely that one can get quite good bounds on relatively short exponential sums when the modulus ${q}$ is smooth, through the basic technique of Weyl differencing (ultimately based on the Cauchy-Schwarz inequality, and also related to the van der Corput lemma in equidistribution theory). Improvements in the case of smooth moduli have appeared before in the literature (e.g. in this paper of Heath-Brown, paper of Graham and Ringrose, this later paper of Heath-Brown, this paper of Chang, or this paper of Goldmakher); the arguments here are particularly close to that of the first paper of Heath-Brown. It now also appears that further optimisation of this Weyl differencing trick could lead to noticeable improvements in the numerology for the polymath8 project, so I am devoting this post to explaining this trick further.

To illustrate the method, let us begin with the classical problem in analytic number theory of estimating an incomplete character sum

$\displaystyle \sum_{M+1 \leq n \leq M+N} \chi(n)$

where ${\chi}$ is a primitive Dirichlet character of some conductor ${q}$, ${M}$ is an integer, and ${N}$ is some quantity between ${1}$ and ${q}$. Clearly we have the trivial bound

$\displaystyle |\sum_{M+1 \leq n \leq M+N} \chi(n)| \leq N; \ \ \ \ \ (1)$

we also have the classical Pólya-Vinogradov inequality

$\displaystyle |\sum_{M+1 \leq n \leq M+N} \chi(n)| \ll q^{1/2} \log q. \ \ \ \ \ (2)$

This latter inequality gives improvements over the trivial bound when ${N}$ is much larger than ${q^{1/2}}$, but not for ${N}$ much smaller than ${q^{1/2}}$. The Pólya-Vinogradov inequality can be deduced via a little Fourier analysis from the completed exponential sum bound

$\displaystyle | \sum_{n \in {\bf Z}/q{\bf Z}} \chi(n) e_q( an )| \ll q^{1/2}$

for any ${a \in {\bf Z}/q{\bf Z}}$, where ${e_q(n) :=e^{2\pi i n/q}}$. (In fact, from the classical theory of Gauss sums, this exponential sum is equal to ${\tau(\chi) \overline{\chi(a)}}$ for some complex number ${\tau(\chi)}$ of norm ${\sqrt{q}}$.)

In the case when ${q}$ is a prime, improving upon the above two inequalities is an important but difficult problem, with only partially satisfactory results so far. To give just one indication of the difficulty, the seemingly modest improvement

$\displaystyle |\sum_{M+1 \leq n \leq M+N} \chi(n)| \ll p^{1/2} \log \log p$

to the Pólya-Vinogradov inequality when ${q=p}$ was a prime required a 14-page paper in Inventiones by Montgomery and Vaughan to prove, and even then it was only conditional on the generalised Riemann hypothesis! See also this more recent paper of Granville and Soundararajan for an unconditional variant of this result in the case that ${\chi}$ has odd order.

Another important improvement is the Burgess bound, which in our notation asserts that

$\displaystyle |\sum_{M+1 \leq n \leq M+N} \chi(n)| \lessapprox N^{1-1/r} q^{\frac{r+1}{4r^2}} \ \ \ \ \ (3)$

for any fixed integer ${r \geq 2}$, assuming that ${q}$ is square-free (for simplicity) and of polynomial size; see this previous post for a discussion of the Burgess argument. This is non-trivial for ${N}$ as small as ${q^{1/4+o(1)}}$.

In the case when ${q}$ is prime, there has been very little improvement to the Burgess bound (or its Fourier dual, which can give bounds for ${N}$ as large as ${q^{3/4-o(1)}}$) in the last fifty years; an improvement to the exponents in (3) in this case (particularly anything that gave a power saving for ${N}$ below ${q^{1/4}}$) would in fact be rather significant news in analytic number theory.

However, in the opposite case when ${q}$ is smooth – that is to say, all of its factors are much smaller than ${q}$ – then one can do better than the Burgess bound in some regimes. This fact has been observed in several places in the literature (in particular, in the papers of Heath-Brown, Graham-Ringrose, Chang, and Goldmakher mentioned previously), but also turns out to (implicitly) be a key insight in Zhang’s paper on bounded prime gaps. In the case of character sums, one such improved estimate (closely related to Theorem 2 of the Heath-Brown paper) is as follows:

Proposition 1 Let ${q}$ be square-free with a factorisation ${q = q_1 q_2}$ and of polynomial size, and let ${M,N}$ be integers with ${1 \leq N \leq q}$. Then for any primitive character ${\chi}$ with conductor ${q}$, one has

$\displaystyle | \sum_{M+1 \leq n \leq M+N} \chi(n) | \lessapprox N^{1/2} q_1^{1/2} + N^{1/2} q_2^{1/4}.$

This proposition is particularly powerful when ${q}$ is smooth, as this gives many factorisations ${q = q_1 q_2}$ with the ability to specify ${q_1,q_2}$ with a fair amount of accuracy. For instance, if ${q}$ is ${y}$-smooth (i.e. all prime factors are at most ${y}$), then by the greedy algorithm one can find a divisor ${q_1}$ of ${q}$ with ${y^{-2/3} q^{1/3} \leq q_1 \leq y^{1/3} q^{1/3}}$; if we set ${q_2 := q/q_1}$, then ${y^{-1/3} q^{2/3} \leq q_2 \leq y^{2/3} q^{2/3}}$, and the above proposition then gives

$\displaystyle | \sum_{M+1 \leq n \leq M+N} \chi(n) | \lessapprox y^{1/6} N^{1/2} q^{1/6}$

which can improve upon the Burgess bound when ${y}$ is small. For instance, if ${N = q^{1/2}}$, then this bound becomes ${\lessapprox y^{1/6} q^{5/12}}$; in contrast the Burgess bound only gives ${\lessapprox q^{7/16}}$ for this value of ${N}$ (using the optimal choice ${r=2}$ for ${r}$), which is inferior for ${y < q^{1/8}}$.

The hypothesis that ${q}$ be squarefree may be relaxed, but for applications to the Polymath8 project, it is only the squarefree moduli that are relevant.

Proof: If ${N \ll q_1}$ then the claim follows from the trivial bound (1), while for ${N \gg q_2}$ the claim follows from (2). Hence we may assume that

$\displaystyle q_1 < N < q_2.$

We use the method of Weyl differencing, the key point being to difference in multiples of ${q_1}$.

Let ${K := \lfloor N/q_1 \rfloor}$, thus ${K \geq 1}$. For any ${1 \leq k \leq K}$, we have

$\displaystyle \sum_{M+1 \leq n \leq M+N} \chi(n) = \sum_n 1_{[M+1,M+N]}(n+kq_1) \chi(n+kq_1)$

and thus on averaging

$\displaystyle \sum_{M+1 \leq n \leq M+N} \chi(n) = \frac{1}{K} \sum_n \sum_{k=1}^K 1_{[M+1,M+N]}(n+kq_1) \chi(n+kq_1). \ \ \ \ \ (4)$

By the Chinese remainder theorem, we may factor

$\displaystyle \chi(n) = \chi_1(n) \chi_2(n)$

where ${\chi_1,\chi_2}$ are primitive characters of conductor ${q_1,q_2}$ respectively. As ${\chi_1}$ is periodic of period ${q_1}$, we thus have

$\displaystyle \chi(n+kq_1) = \chi_1(n) \chi_2(n+kq_2)$

and so we can take ${\chi_1}$ out of the inner summation of the right-hand side of (4) to obtain

$\displaystyle \sum_{M+1 \leq n \leq M+N} \chi(n) = \frac{1}{K} \sum_n \chi_1(n) \sum_{k=1}^K 1_{[M+1,M+N]}(n+kq_1) \chi_2(n+kq_1)$

and hence by the triangle inequality

$\displaystyle |\sum_{M+1 \leq n \leq M+N} \chi(n)| \leq \frac{1}{K} \sum_n |\sum_{k=1}^K 1_{[M+1,M+N]}(n+kq_1) \chi_2(n+kq_1)|.$

Note how the characters on the right-hand side only have period ${q_2}$ rather than ${q=q_1 q_2}$. This reduction in the period is ultimately the source of the saving over the Pólya-Vinogradov inequality.

Note that the inner sum vanishes unless ${n \in [M+1-Kq_1,M+N]}$, which is an interval of length ${O(N)}$ by choice of ${K}$. Thus by Cauchy-Schwarz one has

$\displaystyle | \sum_{M+1 \leq n \leq M+N} \chi(n) | \ll$

$\displaystyle \frac{N^{1/2}}{K} (\sum_n |\sum_{k=1}^K 1_{[M+1,M+N]}(n+kq_1) \chi_2(n+kq_1)|^2)^{1/2}.$

We expand the right-hand side as

$\displaystyle \frac{N^{1/2}}{K} |\sum_{1 \leq k,k' \leq K} \sum_n$

$\displaystyle 1_{[M+1,M+N]}(n+kq_1) 1_{[M+1,M+N]}(n+k'q_1) \chi_2(n+kq_1) \overline{\chi_2(n+k'q_1)}|^{1/2}.$

We first consider the diagonal contribution ${k=k'}$. In this case we use the trivial bound ${O(N)}$ for the inner summation, and we soon see that the total contribution here is ${O( K^{-1/2} N ) = O( N^{1/2}q_1^{1/2} )}$.

Now we consider the off-diagonal case; by symmetry we can take ${k < k'}$. Then the indicator functions ${1_{[M+1,M+N]}(n+kq_1) 1_{[M+1,M+N]}(n+k'q_1)}$ restrict ${n}$ to the interval ${[M+1-kq_1, M+N-k'q_1]}$. On the other hand, as a consequence of the Weil conjectures for curves one can show that

$\displaystyle |\sum_{n \in {\bf Z}/q_2{\bf Z}} \chi_2(n+kq_1) \overline{\chi_2(n+k'q_1)} e_{q_2}(an)| \lessapprox q_2^{1/2} (k-k',q_2)^{1/2}$

for any ${a \in {\bf Z}/q_2{\bf Z}}$; indeed one can use the Chinese remainder theorem and the square-free nature of ${q_2}$ to reduce to the case when ${q_2}$ is prime, in which case one can apply (for instance) the original paper of Weil to establish this bound, noting also that ${q_1}$ and ${q_2}$ are coprime since ${q}$ is squarefree. Applying the method of completion of sums (or the Parseval formula), this shows that

$\displaystyle |\sum_n 1_{[M+1,M+N]}(n+kq_1) 1_{[M+1,M+N]}(n+k'q_1) \chi_2(n+kq_1) \overline{\chi_2(n+k'q_1)}|$

$\displaystyle \lessapprox q_2^{1/2} (k-k',q_2)^{1/2}.$

Summing in ${k,k'}$ (using Lemma 5 from this previous post) we see that the total contribution to the off-diagonal case is

$\displaystyle \lessapprox \frac{N^{1/2}}{K} ( K^2 q_2^{1/2} )^{1/2}$

which simplifies to ${\lessapprox N^{1/2} q_2^{1/4}}$. The claim follows. $\Box$

A modification of the above argument (using more complicated versions of the Weil conjectures) allows one to replace the summand ${\chi(n)}$ by more complicated summands such as ${\chi(f(n)) e_q(g(n))}$ for some polynomials or rational functions ${f,g}$ of bounded degree and obeying a suitable non-degeneracy condition (after restricting of course to those ${n}$ for which the arguments ${f(n),g(n)}$ are well-defined). We will not detail this here, but instead turn to the question of estimating slightly longer exponential sums, such as

$\displaystyle \sum_{1 \leq n \leq N} e_{d_1}( \frac{c_1}{n} ) e_{d_2}( \frac{c_2}{n+l} )$

where ${N}$ should be thought of as a little bit larger than ${(d_1d_2)^{1/2}}$.

This is the final continuation of the online reading seminar of Zhang’s paper for the polymath8 project. (There are two other continuations; this previous post, which deals with the combinatorial aspects of the second part of Zhang’s paper, and this previous post, that covers the Type I and Type II sums.) The main purpose of this post is to present (and hopefully, to improve upon) the treatment of the final and most innovative of the key estimates in Zhang’s paper, namely the Type III estimate.

The main estimate was already stated as Theorem 17 in the previous post, but we quickly recall the relevant definitions here. As in other posts, we always take ${x}$ to be a parameter going off to infinity, with the usual asymptotic notation ${O(), o(), \ll}$ associated to this parameter.

Definition 1 (Coefficient sequences) A coefficient sequence is a finitely supported sequence ${\alpha: {\bf N} \rightarrow {\bf R}}$ that obeys the bounds

$\displaystyle |\alpha(n)| \ll \tau^{O(1)}(n) \log^{O(1)}(x) \ \ \ \ \ (1)$

for all ${n}$, where ${\tau}$ is the divisor function.

• (i) If ${\alpha}$ is a coefficient sequence and ${a\ (q) = a \hbox{ mod } q}$ is a primitive residue class, the (signed) discrepancy ${\Delta(\alpha; a\ (q))}$ of ${\alpha}$ in the sequence is defined to be the quantity

$\displaystyle \Delta(\alpha; a \ (q)) := \sum_{n: n = a\ (q)} \alpha(n) - \frac{1}{\phi(q)} \sum_{n: (n,q)=1} \alpha(n). \ \ \ \ \ (2)$

• (ii) A coefficient sequence ${\alpha}$ is said to be at scale ${N}$ for some ${N \geq 1}$ if it is supported on an interval of the form ${[(1-O(\log^{-A_0} x)) N, (1+O(\log^{-A_0} x)) N]}$.
• (iii) A coefficient sequence ${\alpha}$ at scale ${N}$ is said to be smooth if it takes the form ${\alpha(n) = \psi(n/N)}$ for some smooth function ${\psi: {\bf R} \rightarrow {\bf C}}$ supported on ${[1-O(\log^{-A_0} x), 1+O(\log^{-A_0} x)]}$ obeying the derivative bounds

$\displaystyle \psi^{(j)}(t) = O( \log^{j A_0} x ) \ \ \ \ \ (3)$

for all fixed ${j \geq 0}$ (note that the implied constant in the ${O()}$ notation may depend on ${j}$).

For any ${I \subset {\bf R}}$, let ${{\mathcal S}_I}$ denote the square-free numbers whose prime factors lie in ${I}$. The main result of this post is then the following result of Zhang:

Theorem 2 (Type III estimate) Let ${\varpi, \delta > 0}$ be fixed quantities, and let ${M, N_1, N_2, N_3 \gg 1}$ be quantities such that

$\displaystyle x \ll M N_1 N_2 N_3 \ll x$

and

$\displaystyle N_1 \gg N_2, N_3$

and

$\displaystyle N_1^4 N_2^4 N_3^5 \gg x^{4+16\varpi+\delta+c}$

for some fixed ${c>0}$. Let ${\alpha, \psi_1, \psi_2, \psi_3}$ be coefficient sequences at scale ${M,N_1,N_2,N_3}$ respectively with ${\psi_1,\psi_2,\psi_3}$ smooth. Then for any ${I \subset [1,x^\delta]}$ we have

$\displaystyle \sum_{q \in {\mathcal S}_I: q< x^{1/2+2\varpi}} \sup_{a \in ({\bf Z}/q{\bf Z})^\times} |\Delta(\alpha \ast \beta; a)| \ll x \log^{-A} x.$

In fact we have the stronger “pointwise” estimate

$\displaystyle |\Delta(\alpha \ast \psi_1 \ast \psi_2 \ast \psi_3; a)| \ll x^{-\epsilon} \frac{x}{q} \ \ \ \ \ (4)$

for all ${q \in {\mathcal S}_I}$ with ${q < x^{1/2+2\varpi}}$ and all ${a \in ({\bf Z}/q{\bf Z})^\times}$, and some fixed ${\epsilon>0}$.

(This is very slightly stronger than previously claimed, in that the condition ${N_2 \gg N_3}$ has been dropped.)

It turns out that Zhang does not exploit any averaging of the ${\alpha}$ factor, and matters reduce to the following:

Theorem 3 (Type III estimate without ${\alpha}$) Let ${\delta > 0}$ be fixed, and let ${1 \ll N_1, N_2, N_3, d \ll x^{O(1)}}$ be quantities such that

$\displaystyle N_1 \gg N_2, N_3$

and

$\displaystyle d \in {\mathcal S}_{[1,x^\delta]}$

and

$\displaystyle N_1^4 N_2^4 N_3^5 \gg d^8 x^{\delta+c}$

for some fixed ${c>0}$. Let ${\psi_1,\psi_2,\psi_3}$ be smooth coefficient sequences at scales ${N_1,N_2,N_3}$ respectively. Then we have

$\displaystyle |\Delta(\psi_1 \ast \psi_2 \ast \psi_3; a)| \ll x^{-\epsilon} \frac{N_1 N_2 N_3}{d}$

for all ${a \in ({\bf Z}/d{\bf Z})^\times}$ and some fixed ${\epsilon>0}$.

Let us quickly see how Theorem 3 implies Theorem 2. To show (4), it suffices to establish the bound

$\displaystyle \sum_{n = a\ (q)} \alpha \ast \psi_1 \ast \psi_2 \ast \psi_3(n) = X + O( x^{-\epsilon} \frac{x}{q} )$

for all ${a \in ({\bf Z}/q{\bf Z})^\times}$, where ${X}$ denotes a quantity that is independent of ${a}$ (but can depend on other quantities such as ${\alpha,\psi_1,\psi_2,\psi_3,q}$). The left-hand side can be rewritten as

$\displaystyle \sum_{b \in ({\bf Z}/q{\bf Z})^\times} \sum_{m = b\ (q)} \alpha(m) \sum_{n = a/b\ (q)} \psi_1 \ast \psi_2 \ast \psi_3(n).$

From Theorem 3 we have

$\displaystyle \sum_{n = a/b\ (q)} \psi_1 \ast \psi_2 \ast \psi_3(n) = Y + O( x^{-\epsilon} \frac{N_1 N_2 N_3}{q} )$

where the quantity ${Y}$ does not depend on ${a}$ or ${b}$. Inserting this asymptotic and using crude bounds on ${\alpha}$ (see Lemma 8 of this previous post) we conclude (4) as required (after modifying ${\epsilon}$ slightly).

It remains to establish Theorem 3. This is done by a set of tools similar to that used to control the Type I and Type II sums:

• (i) completion of sums;
• (ii) the Weil conjectures and bounds on Ramanujan sums;
• (iii) factorisation of smooth moduli ${q \in {\mathcal S}_I}$;
• (iv) the Cauchy-Schwarz and triangle inequalities (Weyl differencing).

The specifics are slightly different though. For the Type I and Type II sums, it was the classical Weil bound on Kloosterman sums that were the key source of power saving; Ramanujan sums only played a minor role, controlling a secondary error term. For the Type III sums, one needs a significantly deeper consequence of the Weil conjectures, namely the estimate of Bombieri and Birch on a three-dimensional variant of a Kloosterman sum. Furthermore, the Ramanujan sums – which are a rare example of sums that actually exhibit better than square root cancellation, thus going beyond even what the Weil conjectures can offer – make a crucial appearance, when combined with the factorisation of the smooth modulus ${q}$ (this new argument is arguably the most original and interesting contribution of Zhang).

I’m closing my series of articles for the Princeton Companion to Mathematics with my article on “Ricci flow“. Of course, this flow on Riemannian manifolds is now very well known to mathematicians, due to its fundamental role in Perelman’s celebrated proof of the Poincaré conjecture. In this short article, I do not focus on that proof, but instead on the more basic questions as to what a Riemannian manifold is, what the Ricci curvature tensor is on such a manifold, and how Ricci flow qualitatively changes the geometry (and with surgery, the topology) of such manifolds over time.

I’ve saved this article for last, in part because it ties in well with my upcoming course on Perelman’s proof which will start in a few weeks (details to follow soon).

The last external article for the PCM that I would like to point out here is Brian Osserman‘s article on the Weil conjectures, which include the “Riemann hypothesis over finite fields” that was famously solved by Deligne. These (now solved) conjectures, which among other things gives some quite precise control on the number of points in an algebraic variety over a finite field, were (and continue to be) a major motivating force behind much of modern arithmetic and algebraic geometry.