Lax descent and foliated Verdier duality

Abstract

Higher categorical methods allow us to view recollements as lax descent data and build a corresponding descent theory governed by an $\infty$-bicategorical theory of limits. We demonstrate some concrete innovations brought along by these new techniques in the classical context of constructible and perverse sheaves on stratified spaces. Our first application uses partially (op)lax descent to recover the famous result of Beilinson telling us how to glue perverse sheaves in terms of vanishing and nearby cycles. The remainder of the article is concerened with establishing a uniform view on a series of classification results for perverse sheaves culminating in recent work of Kapranov-Schechtman. Namely, we use lax descent to establish a ``foliated'' variant of Verdier duality which, informally speaking, identifies the derived constructible category of a suitably foliated space with sections of a categorical cosheaf on the leaf space whose stalks are the categories of sheaves along the leaves. Again, this is obtained as an application of lax descent, and seems to provide a reasonable framework to formalize the idea of ``mixed sheaf-cosheaf objects'' promoted by Kapranov and Schechtman.