(en) Explicit formulae are obtained, giving the number of independent matrices which commute with the matrices S and T describing the modular transformations of the SU(N) affine characters.
Ruelle, P. (1990). Dimension of the commutant for the SU(N) affine algebra. Communications in Mathematical Physics, 133, 181-196. https://doi.org/10.1007/BF02096560 (Original work published 1990)