現代数学の系譜工学物理雑談古典ガロア理論も読む79

[過去ﾛｸﾞ] 現代数学の系譜工学物理雑談古典ガロア理論も読む79 (1002ﾚｽ)
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336: 現代数学の系譜 雑談 古典ガロア理論も読む ◆e.a0E5TtKE  [] 2019/12/07(土) 19:38:50.21 ID:H2e5WMAT

>>315
二番煎じでも、先の定理を拡張したり、一般化すれば、評価はまた変わる
Weil conjecturesの”second proof”

https://en.wikipedia.org/wiki/Weil_conjectures
Weil conjectures
(抜粋)
Deligne's first proof of the remaining third Weil conjecture (the "Riemann hypothesis conjecture") used the following steps:

Use of Lefschetz pencils
・The theory of monodromy of Lefschetz pencils, introduced for complex varieties (and ordinary cohomology) by Lefschetz (1924), and extended by Grothendieck (1972) and Deligne & Katz (1973) to l-adic cohomology, relates the cohomology of V to that of its fibers.
The relation depends on the space Ex of vanishing cycles, the subspace of the cohomology Hd?1(Vx) of a non-singular fiber Vx, spanned by classes that vanish on singular fibers.
・The Leray spectral sequence relates the middle cohomology group of V to the cohomology of the fiber and base.
c(U,E), where U is the points the projective line with non-singular fibers, and j is the inclusion of U into the projective line, and E is the sheaf with fibers the spaces Ex of vanishing cycles.

The key estimate
The heart of Deligne's proof is to show that the sheaf E over U is pure, in other words to find the absolute values of the eigenvalues of Frobenius on its stalks.
This is done by studying the zeta functions of the even powers Ek of E and applying Grothendieck's formula for the zeta functions as alternating products over cohomology groups.
The crucial idea of considering even k powers of E was inspired by the paper Rankin (1939), who used a similar idea with k=2 for bounding the Ramanujan tau function. Langlands (1970, section 8) pointed out that a generalization of Rankin's result for higher even values of k would imply the Ramanujan conjecture,
and Deligne realized that in the case of zeta functions of varieties, Grothendieck's theory of zeta functions of sheaves provided an analogue of this generalization.
つづく

http://rio2016.5ch.net/test/read.cgi/math/1573769803/336

337: 現代数学の系譜 雑談 古典ガロア理論も読む ◆e.a0E5TtKE  [] 2019/12/07(土) 19:39:38.79 ID:H2e5WMAT

>>336

つづき

Completion of the proof
The deduction of the Riemann hypothesis from this estimate is mostly a fairly straightforward use of standard techniques and is done as follows.

Deligne's second proof
Deligne (1980) found and proved a generalization of the Weil conjectures, bounding the weights of the pushforward of a sheaf.
In practice it is this generalization rather than the original Weil conjectures that is mostly used in applications, such as the hard Lefschetz theorem. Much of the second proof is a rearrangement of the ideas of his first proof.
The main extra idea needed is an argument closely related to the theorem of Jacques Hadamard and Charles Jean de la Vallee Poussin, used by Deligne to show that various L-series do not have zeros with real part 1.

Inspired by the work of Witten (1982) on Morse theory, Laumon (1987) found another proof, using Deligne's l-adic Fourier transform, which allowed him to simplify Deligne's proof by avoiding the use of the method of Hadamard and de la Vallee Poussin.
His proof generalizes the classical calculation of the absolute value of Gauss sums using the fact that the norm of a Fourier transform has a simple relation to the norm of the original function.
Kiehl & Weissauer (2001) used Laumon's proof as the basis for their exposition of Deligne's theorem. Katz (2001) gave a further simplification of Laumon's proof, using monodromy in the spirit of Deligne's first proof.
Kedlaya (2006) gave another proof using the Fourier transform, replacing etale cohomology with rigid cohomology.

つづく

http://rio2016.5ch.net/test/read.cgi/math/1573769803/337

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