Extending monoidal structures on fibered categories via embeddings

Abstract

Let $\mathcal{S}$ be a small category, and suppose that we are given a full subcategory $\mathcal{U}$ such that every object of $\mathcal{S}$ can be embedded into some object of $\mathcal{U}$ in the same way as every quasi-projective algebraic variety admits a closed embedding into a smooth one. We show that every monoidal structure on a given $\mathcal{S}$-fibered category satisfying certain natural conditions is completely determined by its restriction to $\mathcal{U}$; in fact, any monoidal structure over $\mathcal{U}$ satisfying similar natural conditions admits an essentially unique extension to the whole of $\mathcal{S}$. For instance, this allows one to recover the unit constraint on the classical constructible derived categories from the abelian categories of perverse sheaves. The same principle applies to morphisms of $\mathcal{S}$-fibered categories and monoidality thereof.