Cyclotomic integers, fusion categories, and subfactors

TL;DR

This paper explores the number theoretic properties of dimensions in fusion categories, providing classifications of possible dimensions and principal graphs of subfactors, with implications for the structure and classification of these mathematical objects.

Contribution

It offers a complete list of possible Frobenius-Perron dimensions in a specific interval and proves a finiteness result for families of graphs related to subfactors.

Findings

Classified all possible object dimensions in (2, 76/33).

Constructed a new fusion category realizing the minimal dimension.

Proved finiteness of certain graph families related to subfactors.

Abstract

Dimensions of objects in fusion categories are cyclotomic integers, hence number theoretic results have implications in the study of fusion categories and finite depth subfactors. We give two such applications. The first application is determining a complete list of numbers in the interval (2, 76/33) which can occur as the Frobenius-Perron dimension of an object in a fusion category. The smallest number on this list is realized in a new fusion category which is constructed in the appendix written by V. Ostrik, while the others are all realized by known examples. The second application proves that in any family of graphs obtained by adding a 2-valent tree to a fixed graph, either only finitely many graphs are principal graphs of subfactors or the family consists of the A_n or D_n Dynkin diagrams. This result is effective, and we apply it to several families arising in the classification of subfactors of index less then 5.