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Hopf Algebras and Congruence Subgroups

The authors prove that the kernel of the action of the modular group on the centre of a semisimple factorisable Hopf algebra is a congruence subgroup whenever this action is linear. If the action is only projective, they show that the projective kernel is a congruence subgroup. To do this, they introduce a class of generalised Frobenius-Schur indicators and endow it with an action of the modular group that is compatible with the orig...