Congruence Property In Conformal Field Theory
  The congruence subgroup property is established for the modular representations associated to any modular tensor category. This result is used to prove that the kernel of the representation of the modular group on the conformal blocks of any rational, C_2-cofinite vertex operator algebra is a congruence subgroup. In particular, the q-character of each irreducible module is a modular function on the same congruence subgroup. The Galois symmetry of the modular representations is obtained and the order of the anomaly for those modular categories satisfying some integrality conditions is determined. 
Chongying Dong, Xingjun Lin, and Siu-Hung Ng
math.QA hep-th math-ph math.CT math.MP math.RT