Splitting field for a group not implies splitting field for every subgroup
- Splitting not implies sufficiently large: This is essentially equivalent because sufficiently large implies implies splitting for every subquotient and splitting field for every subgroup implies sufficiently large.
Consider symmetric group:S3, the symmetric group of degree three. The rational numbers are a splitting field for this group. However, they are not a splitting field for the alternating group of degree three, because there exists a character of this group that is not realizable over the rationals.
More generally, by Cayley's theorem, any finite group can be embedded as a subgroup in a symmetric group of finite degree, and symmetric groups are rational-representation groups, i.e., the rational numbers are a splitting field for every symmetric group. On the other hand, the rational numbers are not a splitting field for most finite groups, giving a plethora of counterexamples.