# Perverse sheaf

The mathematical term perverse sheaves refers to a certain abelian category associated to a topological space X, which may be a real or complex manifold, or a more general topologically stratified space, usually singular. This concept was introduced by Zoghman Mebkhout in his thesis and then further developed by Joseph Bernstein, Alexander Beilinson, Pierre Deligne, and Ofer Gabber (1982) as a formalisation of the Riemann-Hilbert correspondence, which related the topology of singular spaces (intersection homology of Mark Goresky and Robert MacPherson) and the algebraic theory of differential equations (microlocal calculus and holonomic D-modules of Joseph Bernstein, Masaki Kashiwara and Takahira Kawai). It was clear from the outset that perverse sheaves are fundamental mathematical objects at the crossroads of algebraic geometry, topology, analysis and differential equations. They also play an important role in number theory, algebra, and representation theory.

## Preliminary remarks

The name perverse sheaf requires explanation: they are not sheaves in the mathematical (or any other) sense, nor are they perverse.[1] The justification is that perverse sheaves have several features in common with sheaves: they form an abelian category, they have cohomology, and to construct one, it suffices to construct it locally everywhere. The adjective "perverse" originates in the intersection homology theory, and its origin was explained by Goresky (2010).

The Beilinson-Bernstein-Deligne definition of a perverse sheaf proceeds through the machinery of triangulated categories in homological algebra and has very strong algebraic flavour, although the main examples arising from Goresky-MacPherson theory are topological in nature. This motivated MacPherson to recast the whole theory in geometric terms on a basis of Morse theory. For many applications in representation theory, perverse sheaves can be treated as a 'black box', a category with certain formal properties.

In the Riemann-Hilbert correspondence, perverse sheaves correspond to holonomic D-modules. This application establishes the notion of perverse sheaf as occurring 'in nature'.

## Definition

A perverse sheaf is an object C of the bounded derived category of sheaves with constructible cohomology on a space X such that the set of points x with

$H^{-i}(j_x^*C)\ne 0$ or $H^{i}(j_x^!C)\ne 0$

has dimension at most 2i, for all i. Here jx is the inclusion map of the point x.

The category of perverse sheaves is an abelian subcategory of the (non-abelian) derived category of sheaves, equal to the core of a suitable t-structure, and is preserved by Verdier duality.

## Notes

1. ^ Les faisceaux pervers n'etant ni des faisceaux, ni pervers, la terminologie requiert une explication. BBD, p. 10