Frobenius-Schur indicators and exponents of spherical categories

• Autores: Siu-Hung Ng, Peter Schauenburg
• Localización: Advances in mathematics, ISSN 0001-8708, Vol. 211, Nº 1, 2007, págs. 34-71
• Idioma: inglés
• DOI: 10.1016/j.aim.2006.07.017
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• Resumen

  • We obtain two formulae for the higher Frobenius¿Schur indicators: one for a spherical fusion category in terms of the twist of its center and the other one for a modular tensor category in terms of its twist. The first one is a categorical generalization of an analogous result by Kashina, Sommerhäuser, and Zhu for Hopf algebras, and the second one extends Bantay's 2nd indicator formula for a conformal field theory to higher degrees. These formulae imply the sequence of higher indicators of an object in these categories is periodic. We define the notion of Frobenius¿Schur (FS-)exponent of a pivotal category to be the global period of all these sequences of higher indicators, and we prove that the FS-exponent of a spherical fusion category is equal to the order of the twist of its center. Consequently, the FS-exponent of a spherical fusion category is a multiple of its exponent, in the sense of Etingof, by a factor not greater than 2. As applications of these results, we prove that the exponent and the dimension of a semisimple quasi-Hopf algebra H have the same prime divisors, which answers two questions of Etingof and Gelaki affirmatively for quasi-Hopf algebras. Moreover, we prove that the FS-exponent of H divides dim(H)4. In addition, if H is a group-theoretic quasi-Hopf algebra, the FS-exponent of H divides dim(H)2, and this upper bound is shown to be tight