Mittagsseminar zur Arithmetik: Gereon Quick (Universität Trondheim): Massey products and formality for real projective groups
Tuesday, 20.05.2025 10:15 im Raum SRZ 216/217
For a profinite group G and a prime number p, let C?(G, Fp) be the differential
graded algebra of continuous cochains on G with Fp-coefficients. If G = GF is the
absolute Galois group of a field F which contains a primitive pth root of unity,
the structure of the associated cohomology algebra H?(GF , Fp) is described in elementary
terms by the celebrated Norm Residue Theorem of Rost and Voevodsky.
One may then ask whether C?(GF , Fp) itself can be described in elementary terms
as well. For example, one may ask whether C?(GF , Fp) is formal, i.e., is quasiisomorphic
as a differential graded algebra to its cohomology algebra. Formality
is a rather strong property and implies, for example, the vanishing of all Massey
products. While there are many examples of non-vanishing Massey products in
arithmetic, Hopkins and Wickelgren showed that all triple Massey products of degree
one classes in the mod 2-Galois cohomology of global fields of characteristic
different from 2 vanish whenever they are defined. They therefore asked whether the
mod 2-cohomology algebra of such fields is formal. Min´a?c and T?an then showed the
vanishing of mod 2-triple Massey products for all fields and formulated the Massey
vanishing conjecture. This conjecture has inspired a lot of activity in recent years,
and it is now known to be true in many cases. The Hopkins?Wickelgren formality
question, however, has a negative answer in general, as Positselski showed that
there are local fields which provide counterexamples.
In my talk, I will report on joint work with Ambrus P´al in which we describe
the first nontrivial family of absolute Galois groups for which C?(GF , Fp) is indeed
formal. In fact, we prove that the mod p-cohomology algebra H?(G, Fp) of any real
projective profinite group G is intrinsically formal for all primes p. By the work
of Haran and Jarden, real projective profinite groups can be characterized as the
absolute Galois groups of fields F with virtual cohomological dimension at most
one.
Angelegt am 15.05.2025 von Heike Harenbrock
Geändert am 15.05.2025 von Heike Harenbrock
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