Cyclic homology has recently revealed its potential in
relation to the description of Serre's Archimedean local factors in the
Hasse-Weil L-function of an arithmetic variety as shown in the paper by
A. Connes and C. Consani : Cyclic homology, Serre's local factors and the lambda-operations.
The elaboration of this topic constitutes one of the two leading themes
of the course that AC is developing at the Collège de France this year.
Cyclic cohomology was introduced and widely publicized in 1981 as an
essential tool in noncommutative differential geometry. The talk Spectral sequence and homology of currents for operator algebras given
by AC at the 1981 Oberwolfach meeting introduced for instance the SBI
long exact sequence and described the cyclic cohomology of the NC torus.
In the context of algebraic geometry instead, the application of cyclic
homology to schemes has a more recent evolution and it has been mainly
promoted in the work of C. Weibel.
In number-theory, there are
three fundamental sources of L-functions 1) arithmetic varieties 2)
geometric Galois representations 3) automorphic representations.
To
a smooth, projective variety X defined over a number field K,
corresponds its Hasse--Weil zeta function which is the alternate product
of the factors $L(H^m,s)$ attached to the Galois representation on
the $\ell$-adic \'etale cohomology $H^m(X_{\bar K},\Q_{\ell})$. The function
$L(H^m,s)$ is defined as an infinite Euler product whose non-archimedean
factors have an immediate geometric meaning at places $\nu$ of K of
good reduction for X (we assume here that $\ell$ does not divide $\nu$), by implementing
the action of the geometric Frobenius on the étale cohomology of the
reduction of $X$ at $\nu$.
At an archimedean place v instead, the
local L-factor is roughly a product of powers of (shifted) Gamma
functions whose definition depends upon the Hodge structure on the m-th
Betti cohomology of the complex variety $X_{v}=X\times_K\C$.
The
main result of the above paper states that the alternate product (as
$m$ ranges from $0$ to twice the dimension of X) of Serre's Archimedean
factors is the inverse graded determinant of the action on cyclic
homology of $X$, with coefficients in infinite adeles
$\prod_{v|\infty} K_v$, of the operator $(2\pi)^{-1}(s-\Theta)$, where
$\Theta$ generates the lambda operations which are the analogue in
cyclic homology of the Adams operations in algebraic K-theory.
Several
subtle features ought to be mentioned at this point. One of them is the
nuance between cyclic homology and archimedean cyclic homology (this
is the one taken up in the above result) which parallels and reflects,
in cyclic homology, the difference between reduced and unreduced real
Deligne cohomology (we refer to the paper for more details).
This result promotes the development of the archimedean cyclic homology
as a theory playing a natural role in the theory of motives in
algebraic geometry, in view of its connection to algebraic K-theory by
the regulator maps. Moreover, it also suggests the study of a
generalization of the above result at the non-archimedean places and the
existence of a global Lefschetz formula in cyclic homology.
The
second part of the course will focus on the description of the
Archimedean counterpart of the rings of periods in p-adic Hodge theory
(especially $B_{cris}$ and $B_{dR}$), by elaborating on the
results contained in the recent collaborative paper The universal thickening of the field of real numbers.
Fontaine's rings of p-adic periods play a fundamental role in
arithmetic in view of the comparison theorems relating étale cohomology
with coefficients in p-adic numbers, with the other fundamental
cohomological theories of arithmetic varieties and in particular with
the de-Rham cohomology thus realizing Grothendieck's idea of ``foncteur
mysterieux''.
At a real place, the transposition of the
construction of the rings of p-adic periods yields non-trivial relevant
rings endowed with a canonical one parameter group of automorphisms
which replaces the Frobenius in Fontaine's construction. At a complex
place, this construction produces fundamental algebraic structures whose
applications transcend the realm of arithmetic by producing a natural
framework in which Feynman integrals in quantum field theory should be
understood.
The class will be entirely given as blackboard-chalk talks.
AC and K. Consani.
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