A bound for the Schur index of irreducible representations of finite groups

We construct an optimal bound for the Schur index of irreducible complex representations of finite groups over the field of rational numbers, when only the prime divisors of the order of the group are known. We study relationships with compatible and universally compatible extensions of number fields. We give a simpler proof of the well-known Berman-Yamada bound for the Schur index over the field Q(p). Bibliography: 7 titles.