Brauer characters with cyclotomic field of values

Abstract

It has been shown in an earlier paper [G. Navarro, Pham Huu Tiep, Rational Brauer characters, Math. Ann. 335 (2006) 675–686] that, for any odd prime p , every finite group of even order has a non-trivial rational-valued irreducible p -Brauer character. For p = 2 this statement is no longer true. In this paper we determine the possible non-abelian composition factors of finite groups without non-trivial rational-valued irreducible 2-Brauer characters. We also prove that, if p ≠ q are primes, then any finite group of order divisible by q has a non-trivial irreducible p -Brauer character with values in the cyclotomic field Q ( exp ( 2 π i / q ) ) .