Class function explained

In mathematics, especially in the fields of group theory and representation theory of groups, a class function is a function on a group G that is constant on the conjugacy classes of G. In other words, it is invariant under the conjugation map on G. Such functions play a basic role in representation theory.

Characters

The character of a linear representation of G over a field K is always a class function with values in K. The class functions form the center of the group ring K[''G'']. Here a class function f is identified with the element

\sum _{g} f(g) g .

Inner products

The set of class functions of a group G with values in a field K form a K-vector space. If G is finite and the characteristic of the field does not divide the order of G, then there is an inner product defined on this space defined by

\langle \phi , \psi \rangle = (1)/(|G|) \sum _{g} \phi(g) \overline{\psi(g)} where |G| denotes the order of G and bar is conjugation in the field K. The set of irreducible characters of G forms an orthogonal basis, and if K is a splitting field for G, for instance if K is algebraically closed, then the irreducible characters form an orthonormal basis.

In the case of a compact group and K = C the field of complex numbers, the notion of Haar measure allows one to replace the finite sum above with an integral:

\langle \phi, \psi \rangle = \int _{G} \phi(t) \overline{\psi(t)} dt.

When K is the real numbers or the complex numbers, the inner product is a non-degenerate Hermitian bilinear form.

See also

• Brauer's theorem on induced characters

References

• Jean-Pierre Serre, Linear representations of finite groups, Graduate Texts in Mathematics 42, Springer-Verlag, Berlin, 1977.