How explicit is the Explicit Formula?

Barry Mazur and William Stein

Notes for a talk at the AMS Special Session on Arithmetic Statistics

(Change slides with left and right arrows. Type "m" to see all slides.)

The Explicit Formulas

The "Explicit Formulas" in analytic number theory deal with arithmetically interesting quantities, often given as partial sums-the summands corresponding to primes

$p$ -up to some cutoff value

$X$ . We'll call them "Sums of local data".

Again:

A "Sum of local data" is a sum of contributions for each prime

$p\leq X$ :

${ \delta(X) := \sum_{p\le X}g(p) }$

where the rules of the game require the values

$g(p)$ to be determined by only local considerations at the prime

$p$ .

Sums of Local Data

We will be concentrating on sums of local data attached to elliptic curves without CM over

$\mathbf{Q}$ ,

${ \delta_E(X):=\sum_{p\le X}g_E(p) }$

where the weighting function

${ p \mapsto g_E(p) }$

is a specific function of

$p$ and

$a_E(p)$ , the

$p$ -th Fourier coefficient of the eigenform of weight two parametrizing the elliptic curve.

Weighted Biases

We will be interested in issues of bias.

Our Aim

Examine computations of these biases, following the classical "Explicit Formula," and the work of:

Sarnak, Granville, Rubenstein, Watkins, Martin, Fiorilli, Conrey-Snaith, ...

Sign of $a_E(p)$

Four our elliptic curves $E$ :

ROUGHLY — half the Fourier coefficients

$a_E(p)$ are positive and half negative.

That is: there are roughly as many

$p$ 's for which the number of rational points of

$E$ over

$\mathbf{F}_p$ is

greater than

$p+1$

as there are primes for which it is

less than

$p+1$ .

Sign of the $a_E(p)$ - a table

$\text{Curve}$	$\text{Positive } a_E(p)\text{ for }p<10^7$	$\text{Negative }a_E(p)\text{ for }p<10^7$
11a (rank 0)	332169	332119
32a (rank 0; CM)	166054	166126
37a (rank 1)	332127	332240
389a (rank 2)	332317	332022
5077a (rank 3)	331706	332632

Finer Statistical Issues

So let's study finer statistical issues related to this symmetric distribution. For example, we can ask the raw question: which of these classes of primes are winning the race, and how often?
I.e., what can one say about:

${ \Delta_E(X) = \frac{\log(X)}{\sqrt{X}} \text{ times } }$

${ \#\{p \text{ such that } |E(\mathbf{F}_p)| > p+1\} }$

minus

${ \#\{p \text{ such that } |E(\mathbf{F}_p)| < p+1\}? }$

Equivalently, putting:

${ \gamma_E(p) = \begin{cases} 0 & \text{if $p$ is a bad or supersingular prime for $E$},\\ -1 & \text{if $E$ has more than $p+1$ points rational over $\mathbf{F}_p$},\\ +1 & \text{if less} \end{cases} }$

${ \Delta_E(X) = \frac{\log(X)}{\sqrt{X}} \sum_{p\leq X} \gamma_E(p) }$

Rank 0 curve 11a (

$p<1000$ ):

Graphs of $\Delta_E(X) = \frac{\log(X)}{\sqrt{X}} \sum_{p\leq X}\gamma_E(p)$

Rank 0 curve 11a (

$p < 10^6$ ):

Rank 1 curve 37a (

$p < 10^6$ ):

More graphs of $\Delta_E(X) = \frac{\log(X)}{\sqrt{X}} \sum_{p\leq X}\gamma_E(p)$

Rank 2 curve 389a (

$p < 10^6$ ):

Rank 3 curve 5077a (

$p < 10^6$ ):

$\Delta_E(X): =\sum_{p\le X}\gamma_E(p)$

"Means" and "Percentages of positive (or negative) support"

Recall that to say that

${\delta(X) = \sum_{p\le X}g(X)}$

possesses a limiting distribution $\mu_\delta$ with respect to the multiplicative measure $dX/X$ means that for continuous bounded functions

$f$ on

$\mathbf{R}$ we have:

${ \lim_{X \to {\infty}}\ {\frac{1}{\log(X)}}\int_0^Xf(\delta(x))\frac{dx}{x} = \int_{\mathbf{R}}f(x)d\mu_\delta(x). }$

The mean of

$\delta(X)$ is by definition:

${ {\mathcal E} : = \lim_{X \to {\infty}}\ {\frac{1}{\log(X)}}\int_0^X\delta(x)\frac{dx}{x} = \int_{\mathbf{R}}d\mu_\delta(x). }$

In the work of Sarnak and Fiorilli, another measure for understanding "bias behavior" is given by what one might call the percentage of positive support (relative to the multiplicative measure

$dX/X$ ). Namely:

${ \begin{align*} {\mathcal P} & := \lim {\rm inf}_{X\to \infty}{\frac{1}{\log(X)}}\int_{2\le x \le X; \delta(x)\le 0}dx/x\\ \quad &= \lim {\rm sup}_{X\to \infty}{\frac{1}{\log(X)}}\int_{2\le x \le X; \delta(x)\le 0}dx/x \end{align*} }$

It is indeed a conjecture, in specific instances interesting to us, that these limits

${\mathcal E}$ and

${\mathcal P}$ exist.

(Discuss a beautiful result of Fiorilli about

${\mathcal P}$ )

${ \text{mean of $\delta(x)$} : = \lim_{X \to {\infty}}\ {\frac{1}{\log(X)}}\int_0^X\delta(x)\frac{dx}{x} = \int_{\mathbf{R}}d\mu_\delta(x). }$

More General Weighting Functions

Consider weighting functions

$p\mapsto g_E(p)$ that have the property that:

for all primes $p$ , the number $g_E(p)$ is an odd function of the value $a_E(p)$
the sum of local data
${ \delta_E(X) := \sum_{p\leq X} g_E(p) }$
has—or can be convincingly conjectured to have—a finite mean.

Any such

$p \mapsto g_E(p)$ represents a version of a "bias race".

To illustrate specific features of the "Explicit Formula" we focus on three examples of such races for an elliptic curve

$E$ .

${ \text{mean of } \delta(x) : = \lim_{X \to {\infty}}\ {\frac{1}{\log(X)}}\int_0^X\delta(x) \frac{dx}{x}; \qquad \gamma_E(p) = \begin{cases} 0 & \text{if $p$ is a bad or supersingular prime for $E$},\\ -1 & \text{if $E$ has more than $p+1$ points rational over $\mathbf{F}_p$},\\ +1 & \text{if less} \end{cases} }$

Sums of Local Data

RAW	$\Delta_E(X): = \frac{\log(X)}{\sqrt{X}} \sum_{p\le X}\gamma_E(p)$
MEDIUM-RARE	${\mathcal D}_E(X):= {\frac{\log(X)}{\sqrt X}}\sum_{p \le X}{\frac{a_E(p)}{\sqrt p}}$
WELL-DONE	${D}_E(X):= {\frac{1}{\log(X)}} \sum_{p \le X}{\frac{a_E(p)\log p}{ p}}$

The fun here is that there are clean conjectures for the values of the means (relative to

$dX/X$ )

—i.e., the biases—

of the three "sums of local data" and clean expectations of their variances:

(Use mouse to hover over definition above to see a conjecture.)

$\Delta_E(X): =\frac{\log(X)}{\sqrt{X}} \sum_{p\le X}\gamma_E(p), \quad {\mathcal D}_E(X):= {\frac{\log(X)}{\sqrt X}}\sum_{p \le X}{\frac{a_E(p)}{\sqrt p}} \to 1-2r,\quad {D}_E(X):= {\frac{1}{\log(X)}} \sum_{p \le X}{\frac{a_E(p)\log p}{ p}} \to -r$

Comments

The (conjectured) distinction in the variances of the three formats:
- The raw data has infinite variance
- The medium-rare and well-done data have finite variance
The numbers
${n\mapsto r_E(n) = \text{ the order of vanishing of }L(\text{symm}^n f_E, s) \text{ at }s=1/2}$
(for $n$ odd) conjecturally determine all biases!
We have the beginnings of some data for those numbers, $n\mapsto r_E(n)$ , but nothing systematic.
And no firm conjectures yet.

Numerically, instead of simply looking at examples of curves of various ranks, we instead look for curves with interesting

$r_E(n)$ and focus on the mean...

${ \text{mean of $\delta(x)$} : = \lim_{X \to {\infty}}\ {\frac{1}{\log(X)}}\int_0^X\delta(x)\frac{dx}{x} = \int_{\mathbf{R}}d\mu_\delta(x);\qquad r_E(n) = \text{ the order of vanishing of }L(\text{symm}^n f_E, s) \text{ at }s=1/2 }$

For example...

If

$g(t)$ is a continuous function on

$[-1,+1]$ with—appropriately defined—Fourier coefficients

$\{c_n\}_n$ , then the mean of the sum of local data

${ \delta(X) := \sum_{p\leq X} g(a(p)/(2\sqrt{p})) }$

is conjecturally

${ \sum_{n=1}^{\infty} c_n(2 r_E(n) + (-1)^n). }$

Thus

${ \left\{\text{ Means of }\delta(X)'s\right\} \longleftrightarrow \left\{ r_E(n)'s \right\} }$

$E$	rank	RAW mean	MEDIUM mean	$\to 1-2r?$	WELL mean	$\to -r?$
11a	0	0.647	0.598	1	0.155	0
14a	0	0.752	0.554	1	0.114	0
37a	1	-1.412	-1.967	-1	-0.816	-1
43a	1	-0.366	-1.906	-1	-0.792	-1
389a	2	-2.658	-4.293	-3	-1.663	-2
433a	2	-4.055	-4.167	-3	-1.617	-2
5077a	3	-5.228	-6.598	-5	-2.507	-3
11197a	3	-4.428	-6.289	-5	-2.360	-3

raw mean

medium-rare mean

well-done mean

Qualitative look at the Explicit Formula

Sum of local data = the "bias" + Oscillatory term + Error term

Qualitative look at the Explicit Formula

For example, for the Well-done data,

${ D_E(X) := \frac{1}{\log(X)}\sum_{p\leq X} \frac{a_{E}(p)\log p}{p} }$

the Explicit Formula gives

$D_E(X)$ as a sum of three contributions:

${ -r_E + S_E(X) + O(1/\log(X)) }$

where the "Oscillatory term"

$S_E(X)$ is the wild card (even assuming GRH) and we take it to be the limit (

$Y\to\infty$ ) of these generalized trigonometric sums:

${ S_E(X,Y) = \frac{1}{\log(X)} \sum_{|\gamma|\leq Y} \frac{X^{i\gamma}}{i\gamma} }$

(sum over imaginary parts of complex zeros of

$L(f_E,s)$ above

$s=1/2$ )

${ S_E(X,Y) = \frac{1}{\log(X)} \sum_{|\gamma|\leq Y} \frac{X^{i\gamma}}{i\gamma} \quad\text{(sum over zeros)} }$

A Tentative Conjecture

It has been tentatively conjectured that

${ \lim_{X,Y\to\infty} S_E(X,Y) = 0, }$

but for computations it would be good to know something more explicit.

${ r_E(n) = \text{ the order of vanishing of }L(\text{symm}^n f_E, s) \text{ at }s=1/2, \qquad S_E(X,Y) = \frac{1}{\log(X)} \sum_{|\gamma|\leq Y} \frac{X^{i\gamma}}{i\gamma} }$

Three issues needing conjectures, and computations:

What should be conjectured about:

the distribution of the $r_{E}(n)$ 's
the convergence of $\lim_{X,Y\to\infty} S_E(X,Y) = 0$
the conditional biases—and multivariate distributions—related to the zeroes of $L$ -functions of tensor products of symmetric powers of two (or more) automorphic forms