Category: Mathematics

Number Theory Days 2016

As usual, with Spring comes the annual Number Theory Days of EPFL and ETHZ, this time in Zürich during the week-end of April 15 and 16. The website is online, and the poster should be ready very soon (I will update the post when it is…)

The meeting is organized by the Forschungsinstitut für Mathematik, and (again as usual!) there is a certain amount of funding for local expenses made available by FIM for young researchers (graduate students and postdocs). Please register on the FIM web page before March 21 if you are interested!

Is MathOverflow insane?

Since my post contra MathOverflow, already five years ago, I’ve continued watching the site and enjoying many of its mathematical discussions, and seeing myself evolve a bit concerning some of my critical opinions. However, I read today with amazement the discussion that evolved from a question of Richard Stanley on the topic of gravitational waves. I applaud the question, the answer and (among the voices of reason) the comments of Lucia.

The negative comments embody the perfect distillation of the perverse puritanical hair-splitting competition known as “Is this question a good fit for MO?” (to be read in a slightly hysterical voice) that is now what I find most annoying on the site. This is not what mathematics (not even “research” mathematics, that seems to replace here the “pure” mathematics illusion of yesteryears) is about for me. I must confess to finding particularly annoying that some of the most vocal critics (e.g., the pseudonymous “quid”) seem to be people with little actual mathematical contributions and too much time to spend and to write for ever and ever on the finer points of etiquette of a web site as if it were some platonic object to protect from all interlopers.

What would Arnold think of this discussion, where “mathematicians” throw away much (he would say “most”) of the whole history, motivation and insights of their science? Would a question of Kolmogorov on what the brain looks like as graph have passed through the fourches caudines of Signor Quid?

Probabilistic number theory

During the fall semester, besides the Linear Algebra course for incoming Mathematics and Physics students, I was teaching a small course on Probabilistic Number Theory, or more precisely on a few aspects of probabilistic number theory that I find especially enjoyable.

Roughly speaking, I concentrated on a small number of examples of statements of convergence in law of sequences of arithmetically-defined probability measures: (1) the Erdös-Kac Theorem; (2) the Bohr-Jessen-Bagchi distribution theorems for the Riemann zeta function on the right of the critical line ; (3) Selberg’s Theorem on the normal behavior of the Riemann zeta function on the critical line ; (4) my own work with W. Sawin on Kloosterman paths. My goal in each case was to do with as little arithmetic, and as much probability, as possible. The reason for this is that I wanted to get and give as clear a picture as possible of which arithmetic facts are involved in each case.

This led to some strange and maybe pedantic-looking discussions at the beginning. But overall I think this was a useful point of view. I think it went especially well in the discussion of Bagchi’s Theorem, which concerns the “functional” limit theorem for vertical translates of the Riemann zeta function in the critical strip, but to the right of the critical line. The proof I gave is, I think, simpler and better motivated than those I have seen (all sources I know follow Bagchi very closely). In the section on Selberg’s Theorem, I used the recent proof found by Radziwill and Soundararajan, which is very elegant (although I didn’t have much time to cover full details during the class).

I have started writing notes for the course, which can be found here. Note that, as usual, the notes are potentially full of mistakes! But those parts which are written are fairly complete, both from the arithmetic and probabilistic point of view.

Bilinear forms with Kloosterman sums

Philippe Michel, Will Sawin and I have just finished the first draft of a paper where we prove estimates for general bilinear forms of the type
\sum_{m\leq M}\sum_{n\leq N}\alpha_m\beta_n K(mn)
where (1) K(x)=\mathrm{Kl}_k(mn;p) is a (normalized) hyper-Kloosterman sum (for k=2, this is a classical Kloosterman sum) modulo a prime p; and (2) the ranges M and N are such that we have non-trivial bounds even if M=N is a bit smaller than \sqrt{p} in logarithmic scale. In other words, we obtain non-trivial results below the “Pólya-Vinogradov range”.

The basic strategy to get this result is not new: it was devised by Fouvry and Michel a number of years ago (inspired at least in part by earlier work of Friedlander-Iwaniec and by the Vinogradov-Karatsuba-style “shift” method to estimate certain short exponential sums). What was missing (despite the strong motivation provided by applications that were known to follow from such a result, one of which is described in a recent preprint of Blomer, Fouvry, Milicevic, Michel and myself) was a way to prove certain estimates for (complete) sums over finite fields, of the type

\sum_{r\in \mathbf{F}_p} R(r,\lambda_1,b_1,b_2,b_3,b_4)\overline{R(r,\lambda_2,c_1,c_2,c_3,c_4)}\ll p^{3/2}

where

R(r,\lambda,b_1,b_2,b_3,b_4)= \sum_{s\in\mathbf{F}_p}e\Bigl(\frac{\lambda s}{p}\Bigr)K(s(r+b_1))K(s(r+b_2))\overline{K(s(r+b_3))K(s(r+b_4))}

unless the parameters (\lambda_1,\lambda_2, b_i, c_i) are in some “diagonal” positions. And we cannot afford too many diagonal cases…

The main contribution of our paper (much of which comes from the ideas of Will!) is to find a relatively robust approach to such estimates.

This relies, as one can expect, from extensive algebraic-geometric arguments to apply the Riemann Hypothesis over finite fields. In fact, from this point of view, this paper is by far the most complicated I’ve ever been involved in. We use, among other things:

  • The Riemann Hypothesis over finite fields, in its most general version of Deligne — indeed, we use it multiple times;
  • The interpretation of the sum over s (in the sum R above) as itself a trace function of sbome sheaf on the space of parameters (\lambda,r,b_1,b_2,b_3,b_4); this follows from the formalism of étale cohomology, which is also used in many other ways (e.g., to detect irreducibility of sheaves by properties of the top-degree cohomology);
  • A very general version of the Euler-Poincaré characteristic formula in étale cohomology – this comes from SGA5;
  • The formalism and properties of vanishing and nearby cycles in étale cohomology, and in particular their relations with local monodromy representations of sheaves on curves;
  • The global \ell-adic Fourier transform of Deligne as well as the local Fourier transform of Laumon;
  • A special case of the homogeneous Fourier transform of Laumon (which we might be able to avoid, although with an argument involving perverse sheaves);
  • Katz’s theory of Kloosterman and hypergeometric sheaves, in particular with respect to the computation of their geometric monodromy groups (and its implication through the Goursat-Kolchin-Ribet Criterion), but also (and equally importantly) with respect to their local monodromy properties;
  • The diophantine criterion for geometric irreducibility (which is again a case of the Riemann Hypothesis)…

Many of these are results and ideas that I was aware of but had never actually used before, and I learnt a lot by seeing how Will exploited and combined them.  I will try to write a few more posts later to (attempt to) explain and motivate them (and how we use them) from an analytic nunber theorist’s viewpoint.  The theory of vanishing cycles, in particular, should have many more applications in extending the range of applicability of Deligne’s Riemann Hypothesis to problems in analytic number theory.

The paper is dedicated to Henryk Iwaniec, who has been over the years the most eloquent and powerful advocate for a deeper use of the work of Deligne (and Katz and others) in applications to analytic number theory.