The Frobenius–Schur indicator is a concept from representation theory, particularly concerning finite groups and their representations. It provides a way to classify irreducible representations of a finite group with respect to their behavior under certain types of symmetry. In more specific terms, the Frobenius–Schur indicator is defined for an irreducible representation of a finite group \( G \) over a field \( K \) (typically, the complex numbers).
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For an irreducible complex character of a representation, the indicator is for real type, for quaternionic type and for complex type, the latter meaning . It detects the invariant bilinear form on an irreducible representation. For an odd-order group, squaring permutes its elements, so every nontrivial irreducible character has indicator zero by character orthogonality.