Dynkin index

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In mathematics, the Dynkin index


of a representation with highest weight |\lambda| of a compact simple Lie algebra g that has a highest weight \lambda is defined by

 {\rm tr}(t_at_b)= 2x_\lambda g_{ab}

evaluated in the representation |\lambda|. Here t_a are the matrices representing the generators, and g_{ab} is

 {\rm tr}(t_at_b)= 2g_{ab}

evaluated in the defining representation.

By taking traces, we find that

x_{\lambda}=\frac{\dim(|\lambda|)}{2\dim(g)}(\lambda, \lambda +2\rho)

where the Weyl vector

\rho=\frac{1}{2}\sum_{\alpha\in \Delta^+} \alpha

is equal to half of the sum of all the positive roots of g. The expression (\lambda, \lambda +2\rho) is the value quadratic Casimir in the representation |\lambda|. The index x_{\lambda} is always a positive integer.

In the particular case where \lambda is the highest root, meaning that |\lambda| is the adjoint representation, x_{\lambda} is equal to the dual Coxeter number.


  • Philippe Di Francesco, Pierre Mathieu, David Sénéchal, Conformal Field Theory, 1997 Springer-Verlag New York, ISBN 0-387-94785-X