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J.-F. Burnol on the Riemann-Weil Explicit Formula
"I think it is fair to recognize at least 3 important topics:
**Topic 1.** The Riemann (Van Mangoldt, *etc.*) relation between primes and
zeros is considered as an identity between two distributions
Comment: It is a little anachronistic to use the term 'distribution', but
this is the idea. In fact Weil's aim in this regard seems to have been at
first to express 'the most general' Riemann-like relation between primes
and zeros. One can argue whether such a thing exists, and more strongly
still whether it really has been achieved by Weil. Also at least prior work
of Guinand existed at the time. Anyway, with hindsight, Weil notes in the
Comments to his *Oeuvres* that he could as well have restricted himself to
the simplest possible (smooth with compact support) test-functions (see
Topic 3 below).
**Topic 2.** The discovery that the local term of the 'prime-side' of this
equality of distributions, corresponding to a finite prime or to an infinite
prime, 'lives' on the local field, which is the completion of the global
number field at this place. Furthermore these local contributions are
susceptible to being written in quite similar ways, whether the place is
finite or infinite.
Comment (more below): This is clearly where the true innovation of the
paper resides. In my opinion this is why the explicit formula deserves to
be called 'the Riemann-Weil' formula. The usually stated formula (which
only reflects Topic 1) has its key ideas in Riemann, and it is not what I
am talking about here. What I am talking about here includes **a surprising
formula for the conductor of a ramified character**, which Weil, in relation
with a result in Class Field Theory by Tate and Sen, has generalized in his
later 1972 paper. He explains this in his book *Basic Number Theory*
(appendix on the Herbrand distribution). In the 1952 paper, this surprising
formula appears as a *Deus Ex Machina*, nothing is explained about its origin,
except that it is a simple calculation if one wants to check it (which is
indeed simple and elementary, and not spelled out in Weil's paper; it is
left for the attentive reader). What is so striking is that this
(elementarily-checked) formula suggests how to rewrite the contribution of
the infinite places. This, Weil does, with the help of choosing the same
'finite-part' notation for different things, so that in the end everything
looks nice and gentle and all alike at all places. Even when dealing with
archimedean places, this is subtle: if you have, say, a complex place, you
have to show (it is not obvious if you don't know in advance) that a
certain distribution on the positive reals actually comes from the non-zero
complex numbers. The least one can say is that Weil's treatment of the real
and complex places is cumbersome.
**Topic 3.** The Riemann Hypothesis is expressed as a positivity property of
the distribution for which two completely distinct expressions have been
given in Topics 1 and 2: one expression involves directly the zeros, the
other is a distribution on the group of idele classes.
Comment: This last step is completely elementary, and has no arithmetic in
it. It deals with the zeros in the most direct manner, and the positivity
criterion has only to be checked on smooth with compact support
test-functions, as more or less suggested in the Comments to the *Oeuvres*.
And the Finale: Weil mentions that **everything could have been done with a
function field**. The Riemann Hypothesis was proved by himself there, and so
the positivity of the distribution holds in that case. He says this is the
most convincing argument to believe in the truth of RH for number fields
too.
Further comments:
on Topic 3:
The positivity criterion can also be expressed in a more minimal manner, as
in a paper by Li, (*Journal of Number Theory*), and the further work by
Bombieri and Lagarias "On Li's criterion for the Riemann Hypothesis" (*Journal
of Number Theory*, 1999).
on Topic 2:
Shai Haran (paper in *Inventiones*) discovered that more uniformity could be
given to the local terms. For this he interpreted them as additive
convolution and obtained a formula which is identical in all cases. He
especially focused on the semi-group whose infinitesimal generator
corresponds to this local term.
If you forget about the semi-group and just look at the infinitesimal
generator you get the distribution which is Fourier Transform of log(|*x*|)
(note that the Weil's 'Finite Parts' expressions then need an additional
minus sign, but this is also how they appear when you are counting the
zeros positively, and you want to do that in the statement of the
positivity criterion.)
So basically, although he chose not to write it this way, Haran observed
that the local terms of the Explicit Formula were all just additive
convolution with log(|*y*|) (additive Fourier Transform of log(|*x*|)). He
gives explicitly the computation for: a finite prime, not dividing the
conductor, and for the real prime. To be complete, you then need to check
for the complex prime, and **most importantly** for the primes dividing the
conductor. It all works OK, so really Haran has improved upon Weil. But we
have then four distinct necessary computations (not to mention dealing with
the discriminant of the number field.)
And my contribution is this: it is centered on the dilation-invariant
operator log(|*x*|) + log(|*y*|), instead of just log(|*y*|).
Of course for the Explicit Formula, you evaluate log(|*x*|) at 1, so this
gives 0. Still good reasons exist to keep it. My contribution, as published in
my CRAS note from the year 2000, has been to provide a proof which is the same in all cases.
This means you do not formulate the Explicit Formula first on the positive
reals, then as an afterthought realize it did come from the local fields:
you directly jump from the critical line to the local fields."
Jean-Francois Burnol,
Mardi 5 Juin 2001
J.-F. Burnol's homepage
Riemann-Weil
explicit formula page
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