English

Fusion categories between $C \boxtimes D$ and $C * D$

Operator Algebras 2013-08-28 v1 Category Theory Quantum Algebra

Abstract

Given a pair of fusion categories CC and DD, we may form the free product CDC * D and the tensor product CDC \boxtimes D. It is natural to think of the tensor product as a quotient of the free product. What other quotients are possible? When C=D=A2C=D=A_2, there is an infinite family of quotients interpolating between the free product and the tensor product (closely related to the A2n1(1)A_{2n-1}^{(1)} and Dn+2(1)D_{n+2}^{(1)} subfactors at index 4). Bisch and Haagerup discovered one example of such an intermediate quotient when C=A2C=A_2 and D=T2D=T_2, and suggested that there might be another family here. We show that such quotients are characterized by parameters n1n \geq 1 and ω\omega with ω2n=1\omega^{2n}=1. For n=1,2,3n=1,2,3, we show ω\omega must be 1, and construct the corresponding quotient (n=1n=1 is the tensor product, n=2n=2 is the example discovered by Bisch and Haagerup, and n=3n=3 is new). We further show that there are no such quotients for 4n104 \leq n \leq 10. Our methods also apply to the case when C=D=T2C=D=T_2, and we prove similar results there. During the preparation of this manuscript we learnt of an independent result of Liu's on subfactors. With the translation between the subfactor and fusion category settings provided here, it follows there are no such quotients for any n4n \geq 4.

Cite

@article{arxiv.1308.5723,
  title  = {Fusion categories between $C \boxtimes D$ and $C * D$},
  author = {Masaki Izumi and Scott Morrison and David Penneys},
  journal= {arXiv preprint arXiv:1308.5723},
  year   = {2013}
}

Comments

42 pages

R2 v1 2026-06-22T01:15:22.114Z