Linkage and translation for tensor products of representations of simple algebraic groups and quantum groups

Abstract: Let $\mathbf{G}$ be either a simple linear algebraic group over an algebraically closed field of characteristic $\ell>0$ or a quantum group at an $\ell$-th root of unity. We define a tensor ideal of singular $\mathbf{G}$-modules in the category $\mathrm{Rep}(\mathbf{G})$ of finite-dimensional $\mathbf{G}$-modules and study the associated quotient category $\mathrm{\underline{Re}p}(\mathbf{G})$, called the regular quotient. Our main results are a 'linkage principle' and a 'translation principle' for tensor products: Let $\mathrm{\underline{Re}p}_0(\mathbf{G})$ be the essential image in $\mathrm{\underline{Re}p}(\mathbf{G})$ of the principal block of $\mathrm{Rep}(\mathbf{G})$. We first show that $\mathrm{\underline{Re}p}_0(\mathbf{G})$ is closed under tensor products in $\mathrm{\underline{Re}p}(\mathbf{G})$. Then we prove that the monoidal structure of $\mathrm{\underline{Re}p}(\mathbf{G})$ is governed to a large extent by the monoidal structure of $\mathrm{\underline{Re}p}_0(\mathbf{G})$. These results can be combined to give an external tensor product decomposition $\mathrm{\underline{Re}p}(\mathbf{G}) \cong \mathrm{Ver}(\mathbf{G}) \boxtimes \mathrm{\underline{Re}p}_0(\mathbf{G})$, where $\mathrm{Ver}(\mathbf{G})$ denotes the Verlinde category of $\mathbf{G}$.