A lower bound on the essential dimension of PGL4 in characteristic 2.

In the present paper, we provide a lower bound of the essential dimension over a field of positive characteristic via Kato's cohomology group, defined by cokernel of a general Artin-Schreier operator. Combining this with Tignol's result on the second trace form of simple algebras of degree , we show that over a field of characteristic . [ABSTRACT FROM AUTHOR]