# Non weakly-group-theoretical integral fusion category

Discussion in 'Mathematics' started by Sebastien Palcoux, Aug 1, 2020 at 8:03 PM.

1. ### Sebastien PalcouxGuest

Is there an integral fusion category of rank $7$, FPdim $210$ and type $(1,5,5,5,6,7,7)$ with the following fusion rules (or the little $\color{purple}{\text{variation}}$ below)?

$$\scriptsize{\begin{smallmatrix} 1 & 0 & 0 & 0& 0& 0& 0 \\ 0 & 1 & 0 & 0& 0& 0& 0 \\ 0 & 0 & 1 & 0& 0& 0& 0 \\ 0 & 0 & 0 & 1& 0& 0& 0 \\ 0 & 0 & 0 & 0& 1& 0& 0 \\ 0 & 0 & 0 & 0& 0& 1& 0 \\ 0 & 0 & 0 & 0& 0& 0& 1 \end{smallmatrix} , \ \begin{smallmatrix} 0 & 1 & 0 & 0& 0& 0& 0 \\ 1 & 1 & 0 & 1& 0& 1& 1 \\ 0 & 0 & 1 & 0& 1& 1& 1 \\ 0 & 1 & 0 & 0& 1& 1& 1 \\ 0 & 0 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \end{smallmatrix} , \ \begin{smallmatrix} 0 & 0 & 1 & 0& 0& 0& 0 \\ 0 & 0 & 1 & 0& 1& 1& 1 \\ 1 & 1 & 1 & 0& 0& 1& 1 \\ 0 & 0 & 0 & 1& 1& 1& 1 \\ 0 & 1 & 0 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \end{smallmatrix} , \ \begin{smallmatrix} 0 & 0 & 0 & 1& 0& 0& 0 \\ 0 & 1 & 0 & 0& 1& 1& 1 \\ 0 & 0 & 0 & 1& 1& 1& 1 \\ 1 & 0 & 1 & 1& 0& 1& 1 \\ 0 & 1 & 1 & 0& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \end{smallmatrix} , \ \begin{smallmatrix} 0 & 0 & 0 & 0& 1& 0& 0 \\ 0 & 0 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 0 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 0& 1& 1& 1 \\ 1 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 2& 1 \\ 0 & 1 & 1 & 1& 1& 1& 2 \end{smallmatrix} , \ \begin{smallmatrix} 0 & 0 & 0 & 0& 0& 1& 0 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 2& 1 \\ 1 & 1 & 1 & 1& 2& \color{purple}{1}& \color{purple}{2} \\ 0 & 1 & 1 & 1& 1& \color{purple}{2}& \color{purple}{2} \end{smallmatrix} , \ \begin{smallmatrix} 0 & 0 & 0 & 0& 0& 0& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 2 \\ 0 & 1 & 1 & 1& 1& \color{purple}{2}& \color{purple}{2} \\ 1 & 1 & 1 & 1& 2& \color{purple}{2}& \color{purple}{1} \end{smallmatrix}}$$

This is the first known non-trivial simple integral fusion ring. It was found by computation with SageMath (this code giving this output after 2 minutes). Note that there is no finite simple group of order $210$, because the order of a (non-trivial) perfect group is divisible by $4$ (but see this post).

Note that $210 = 2\cdot 3\cdot 5\cdot 7$ and that the above matrices are irreducible; they are also self-dual and commuting, so simultaneously diagonalizable. We deduce the following "formal" character table:

$$\scriptsize \begin{array}{c|c} \text{class}&C_1&C_2&C_3&C_4&C_5&C_6&C_7 \newline \text{size}&1&35&30&30&30&42&42 \newline \hline \chi_1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 \newline \chi_2 & 5 & -1 & -\zeta_7 -\zeta_7^6 & -\zeta_7^5 - \zeta_7^2 & -\zeta_7^4 - \zeta_7^3 & 0 & 0 \newline \chi_3 & 5 & -1 & -\zeta_7^5 - \zeta_7^2 & -\zeta_7^4 - \zeta_7^3 & -\zeta_7 -\zeta_7^6 & 0 & 0 \newline \chi_4 & 5 & -1 & -\zeta_7^4 - \zeta_7^3 & -\zeta_7 -\zeta_7^6 & -\zeta_7^5 - \zeta_7^2 & 0 & 0 \newline \chi_5 & 6 & 0 & -1 & -1 & -1 & 1 & 1 \newline \chi_6 & 7 & 1 & 0 & 0 & 0 & \zeta_5+\zeta_5^4 & \zeta_5^2+\zeta_5^3 \newline \chi_7 & 7 & 1 & 0 & 0 & 0 & \zeta_5^2+\zeta_5^3 & \zeta_5+\zeta_5^4 \newline \end{array}$$

See Schur orthogonality relations and Orbit-Stabilizer theorem together with MR2863455.

Assume the existence of an integral fusion category $\mathcal{C}$ having the above fusion ring.

Theorem: The integral fusion category $\mathcal{C}$ would be not weakly-group-theoretical.
Proof: the fusion ring is simple, so the result follows by MR2735754 Proposition 9.11. because there is no simple group of order $210$. $\square$

Theorem: The integral fusion category $\mathcal{C}$ would be abelian but not braided.
Proof: it is abelian because the above matrices commute. Now, if it's braided, then it can be non-degenerated (i.e. $\mathcal{C}′=Vec$) or degenerated:

• If it's non-degenerated then the contradiction follows by MR3077244 Corollary 6.16.
• Else it's degenerated, then $Vec \subsetneq \mathcal{C}′$, so by simplicity $\mathcal{C}′=\mathcal{C}$, so $\mathcal{C}$ is symmetric, and by Deligne (see MR3077244 Example 4.6 citing MR1106898), $\mathcal{C}≃Rep(G)$ as fusion category (without considering the symmetric structure), with $G$ a finite simple group, contradiction. $\square$

Remark: A pointed fusion category is equivalent to $Vec_{G, \omega}$ with $G$ a finite group and $\omega$ a $3$-cocycle in $Z^3 (G, \mathbb{C}^*)$ as associator. In particular, the fusion ring of a pointed fusion category is trivial.

Aknowledgment: The above theorems were discussed during the workshop on fusion categories (May 2013, IMB Dijon, France). Thanks to Peter Schauenburg for his invitation, and thanks to Eric Rowell, Leonid Vainermann and David Penneys for above references and encouragements.

There is a second integral simple fusion ring of same FPdim, type and rules except a little $\color{purple}{\text{variation}}$ for the $7$-dimensional simple objects (and multiplicity $3$ instead of $2$):

$$\scriptsize{\begin{smallmatrix} 1 & 0 & 0 & 0& 0& 0& 0 \\ 0 & 1 & 0 & 0& 0& 0& 0 \\ 0 & 0 & 1 & 0& 0& 0& 0 \\ 0 & 0 & 0 & 1& 0& 0& 0 \\ 0 & 0 & 0 & 0& 1& 0& 0 \\ 0 & 0 & 0 & 0& 0& 1& 0 \\ 0 & 0 & 0 & 0& 0& 0& 1 \end{smallmatrix} , \ \begin{smallmatrix} 0 & 1 & 0 & 0& 0& 0& 0 \\ 1 & 1 & 0 & 1& 0& 1& 1 \\ 0 & 0 & 1 & 0& 1& 1& 1 \\ 0 & 1 & 0 & 0& 1& 1& 1 \\ 0 & 0 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \end{smallmatrix} , \ \begin{smallmatrix} 0 & 0 & 1 & 0& 0& 0& 0 \\ 0 & 0 & 1 & 0& 1& 1& 1 \\ 1 & 1 & 1 & 0& 0& 1& 1 \\ 0 & 0 & 0 & 1& 1& 1& 1 \\ 0 & 1 & 0 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \end{smallmatrix} , \ \begin{smallmatrix} 0 & 0 & 0 & 1& 0& 0& 0 \\ 0 & 1 & 0 & 0& 1& 1& 1 \\ 0 & 0 & 0 & 1& 1& 1& 1 \\ 1 & 0 & 1 & 1& 0& 1& 1 \\ 0 & 1 & 1 & 0& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \end{smallmatrix} , \ \begin{smallmatrix} 0 & 0 & 0 & 0& 1& 0& 0 \\ 0 & 0 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 0 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 0& 1& 1& 1 \\ 1 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 2& 1 \\ 0 & 1 & 1 & 1& 1& 1& 2 \end{smallmatrix} , \ \begin{smallmatrix} 0 & 0 & 0 & 0& 0& 1& 0 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 2& 1 \\ 1 & 1 & 1 & 1& 2& {\color{purple}{0}}& {\color{purple}{3}} \\ 0 & 1 & 1 & 1& 1& {\color{purple}{3}}& {\color{purple}{1}} \end{smallmatrix} , \ \begin{smallmatrix} 0 & 0 & 0 & 0& 0& 0& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 1 \\ 0 & 1 & 1 & 1& 1& 1& 2 \\ 0 & 1 & 1 & 1& 1& {\color{purple}{3}}& {\color{purple}{1}} \\ 1 & 1 & 1 & 1& 2& {\color{purple}{1}}& {\color{purple}{2}} \end{smallmatrix}}$$

All the properties above hold for this fusion ring. Here is the corresponding formal character table:

$$\scriptsize \begin{array}{c|c} \text{class}&C_1&C_2&C_3&C_4&C_5&C_6&C_7 \newline \text{size}&1&35&30&30&30&70&14 \newline \hline \chi_1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 \newline \chi_2 & 5 & -1 & -\zeta_7 -\zeta_7^6 & -\zeta_7^5 - \zeta_7^2 & -\zeta_7^4 - \zeta_7^3 & 0 & 0 \newline \chi_3 & 5 & -1 & -\zeta_7^5 - \zeta_7^2 & -\zeta_7^4 - \zeta_7^3 & -\zeta_7 -\zeta_7^6 & 0 & 0 \newline \chi_4 & 5 & -1 & -\zeta_7^4 - \zeta_7^3 & -\zeta_7 -\zeta_7^6 & -\zeta_7^5 - \zeta_7^2 & 0 & 0 \newline \chi_5 & 6 & 0 & -1 & -1 & -1 & 1 & 1 \newline \chi_6 & 7 & 1 & 0 & 0 & 0 & 0 & -3 \newline \chi_7 & 7 & 1 & 0 & 0 & 0 & -1 & 2 \newline \end{array}$$

These matrices came from my will of classifying the cyclic subfactors (see this post and MR3708264). I first wondered whether there is an irreducible finite index depth $2$ maximal subfactor which is not a group subfactor, in other words, a finite dimensional Hopf ${\rm C}^{\star}$-algebra (or Kac algebra) nontrivial and without left coideal $\star$-subalgebra (see this post). Such a Kac algebra must be simple. Now a Kac algebra gives a unitary integral fusion category. This algorithm investigates all the (non-trivial) simple integral fusion rings. There are finitely many possibilities for each dimension.
Other examples $\DeclareMathOperator\PSL{PSL} \DeclareMathOperator\Rep{Rep}$
See the Atlas of Fusion Rings, providing thousands of new simple integral fusion rings, by improved brute-force classification, interpolation and isotypical variations. The interpolation is very promising, it should provide infinite families, see this post dedicated to this method: there we show that the first fusion ring above would be the Grothendieck ring of an interpolation called $“\Rep(\PSL(2,6))”$, whereas the second above is just a specific isotypical variation of the first.