$q$-Virasoro modular triple

Inspired by 5d supersymmetric Yang-Mills theories placed on the compact space ${\mathbb{S}}^5$, we propose an intriguing algebraic construction for the $q$-Virasoro algebra. We show that, when multiple $q$-Virasoro “chiral” sectors have to be fused together, a natural $\mathrm{S}\mathrm{L}(3,\mathbb{Z})$ structure arises. This construction, which we call the modular triple, is consistent with the observed triple factorization properties of supersymmetric partition functions derived from localization arguments. We also give a 2d CFT-like construction of the modular triple, and conjecture for the first time a (non-local) Lagrangian formulation for a $q$-Virasoro model, resembling ordinary Liouville theory.