Extending the trace of a pivotal monoidal functor

TL;DR

This paper extends the trace of pivotal monoidal functors from modular tensor categories to tube categories, revealing connections to modular invariants and explaining the A-D-E classification pattern.

Contribution

It introduces a method to extend traces to tube categories and links these to modular invariants and Frobenius algebras, providing new insights into the A-D-E classification.

Findings

Trace extension to tube categories yields candidate modular invariants.

Under certain conditions, the representation forms a haploid, symmetric Frobenius algebra.

Application to Temperley-Lieb categories explains the A-D-E pattern in modular invariants.

Abstract

We consider a pivotal monoidal functor whose domain is a modular tensor category (MTC). We show that the trace of such a functor naturally extends to a representation of the corresponding tube category. As irreducible representations of the tube category are indexed by pairs of simple objects in the underlying MTC, the simple multiplicities of this representation form a candidate modular invariant matrix. In general, this matrix will not be modular invariant, however it will always commute with the T-matrix. Furthermore, under certain additional conditions on the original functor, it is shown that the corresponding representation of the tube category is a haploid, symmetric, commutative Frobenius algebra. Such algebras are known to be connected to modular invariants, in particular a result of Kong and Runkel implies that the matrix of simple multiplicities commutes with the S-matrix if and only if the dimension of the algebra is equal to the dimension of the underlying MTC. Finally, we apply these techniques to certain pivotal monoidal functors arising from module categories over the Temperley-Lieb category and the associated MTC. This provides a novel explanation of the A-D-E pattern appearing in the classification of $ A_1^{(1)} $ modular invariants.