ESSENTIAL DIMENSION OF GENERIC SYMBOLS IN CHARACTERISTIC $p$

KELLY MCKINNIE

Forum of Mathematics, Sigma, Tome 5 (2017)

Résumé

In this article the $p$ -essential dimension of generic symbols over fields of characteristic $p$ is studied. In particular, the $p$ -essential dimension of the length $\ell$ generic $p$ -symbol of degree $n+1$ is bounded below by $n+\ell$ when the base field is algebraically closed of characteristic $p$ . The proof uses new techniques for working with residues in Milne–Kato $p$ -cohomology and builds on work of Babic and Chernousov in the Witt group in characteristic 2. Two corollaries on $p$ -symbol algebras (i.e, degree 2 symbols) result from this work. The generic $p$ -symbol algebra of length $\ell$ is shown to have $p$ -essential dimension equal to $\ell +1$ as a $p$ -torsion Brauer class. The second is a lower bound of $\ell +1$ on the $p$ -essential dimension of the functor $\operatorname{Alg}_{p^{\ell },p}$ . Roughly speaking this says that you will need at least $\ell +1$ independent parameters to be able to specify any given algebra of degree $p^{\ell }$ and exponent $p$ over a field of characteristic $p$ and improves on the previously established lower bound of 3.

Détail

DOI : 10.1017/fms.2017.11