In mathematics, the Chang number of an irreducible representation of a simple complex Lie algebra

Lie algebra

In mathematics, a Lie algebra is an algebraic structure whose main use is in studying geometric objects such as Lie groups and differentiable manifolds. Lie algebras were introduced to study the concept of infinitesimal transformations. The term "Lie algebra" was introduced by Hermann Weyl in the...

 is its dimension modulo 1 + h, where h is the Coxeter number

Coxeter number

In mathematics, the Coxeter number h is the order of a Coxeter element of an irreducible Coxeter group, hence also of a root system or its Weyl group. It is named after H.S.M. Coxeter.-Definitions:...

. Chang numbers are named after , who rediscovered an element of order h + 1 found by .

showed that there is a unique class of regular elements σ of order h + 1, in the complex points of the corresponding Chevalley group. He showed that the trace of σ on an irreducible representation is −1, 0, or +1, and if h + 1 is prime then the trace is congruent to the dimension mod h+1. This implies that the dimension of an irreducible representation is always −1, 0, or +1 mod h + 1 whenever h + 1 is prime.

Examples

In particular, for the exceptional compact Lie groups G₂, F4, E6, E7, and E8 the number h + 1 = 7, 13, 13, 19, 31 is always prime, so the Chang number of an irreducible representation is always +1, 0, or −1.

For example, the first few irreducible representations of G2 (with Coxeter number h = 6) have dimensions 1, 7, 14, 27, 64, 77, 182, 189, 273, 286,...
These are congruent to 1, 0, 0, −1, 1, 0, 0, 0, 0, −1,... mod 7 = h + 1.