Cohomological Invariants

Cohomological Invariants Synopsis

This volume concerns invariants of $G$-torsors with values in mod $p$ Galois cohomology-in the sense of Serre's lectures in the book Cohomological invariants in Galois cohomology-for various simple algebraic groups $G$ and primes $p$. The author determines the invariants for the exceptional groups $F_4$ mod 3, simply connected $E_6$ mod 3, $E_7$ mod 3, and $E_8$ mod 5. He also determines the invariants of $mathrm{Spin}_n$ mod 2 for $n leq 12$ and constructs some invariants of $mathrm{Spin}_{14}$. Along the way, the author proves that certain maps in nonabelian cohomology are surjective. These surjectivities give as corollaries Pfister's results on 10- and 12-dimensional quadratic forms and Rost's theorem on 14-dimensional quadratic forms. This material on quadratic forms and invariants of $mathrm{Spin}_n$ is based on unpublished work of Markus Rost. An appendix by Detlev Hoffmann proves a generalization of the Common Slot Theorem for 2-Pfister quadratic forms.