Invariant derivations and trace bounds

Abstract

About 20 years ago, J-P.~Serre announced a bound on the trace of elements of compact Lie groups under the adjoint representation together with related results, provided indications of his proofs, and invited a better proof. This note provides a new, general method for proving such bounds; uses that method to derive Serre's bounds; gives a second proof of Serre's announced results that (we learned) closely follows his original argument; and provides lower bounds for traces of other representations of compact Lie groups and for Brauer characters of finite groups.

v4 has small improvements to the exposition. v3 re-writes part I, expands other parts of the text, and adds an appendix on finite groups. v2 strengthens the results of part IV