Character table

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This article gives a basic definition in the following area: linear representation theory
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Definition

Let G be a finite group, and k a sufficiently large field for G. The character table of G is a matrix whose rows are indexed by the irreducible representations of G, and columns by the conjugacy classes in G, where the entry in row \rho and column c is the character of the representation \rho on the conjugacy class c.

Facts

Square matrix

There as many conjugacy classes as irreducible representations, so the matrix is a square matrix.

Orthogonality of the rows

Define the weighted character table as the character table with each column multipled by the size of the conjugacy class.

The rows of the weighted character table are orthogonal to each other. Further, the inner product of each row with itself is the cardinality of the group. This in particular shows that the (weighted) characters form an orthogonal basis for the row space and hence for the space of all function spaces. In particular, any class function can be written in a unique way as a linear combination of characters.

Orthogonality of the columns

Since the rows are orthogonal and the inner product of each with itself is the cardinality of the group, the product of the matrix and its transpose is a scalar matrix. Thus, the columns of the weighted character table are orthogonal, which in turn shows that the columns of the original character table are orthogonal.

No ordering of the rows and the columns

As such, there is no canonical way in which we can order the rows (viz irreducible representations) or the columns (viz conjugacy classes). Thus the character table is ambiguous upto both pre- and post-multiplication by permutation matrices.

However, if we are given a conjugacy class-representation bijection, we can use that to identify rows and columns, and hence we can arrange rows and columns in the same order. Now, the character table is ambiguous upto conjugation by a permutation matrix.

This, for instance, is what happens in the case of the symmetric group.