 
Summary: Applications of Homological Algebra Introduction to Perverse Sheaves
Spring 2007 P. Achar
Basic Facts on Sheaves
Definition 1. A sheaf of abelian groups F on a topological space X is the following collection of data:
· for each open set U X, an abelian group F(U), with F() = 0
· if U V , a restriction map V U : F(V ) F(U), with UU = id
such that
(1) (restriction) if U V W, then W U = V U W V
(2) (gluing) if {Vi} is an open covering of U, and we have si F(Vi) such that for all i, j, Vi,ViVj (si) =
Vj ,ViVj (sj), then there exists a unique s F(U) such that U,Vi (s) = si for all i.
If one omits the gluing condition from the above definition, one has a presheaf of abelian groups.
Elements of F(U) are called sections of F over U, and elements of F(X) are called global sections.
One can also define (pre)sheaves of Rmodules, vector spaces, etc.
Notation 2. (U, F) := F(U). sV := V U (s).
Lemma 3. Let F be a sheaf. If {Vi} is an open cover of U, and s, t F(U) are sections such that sVi
= tVi
for all i, then s = t. In particular, if sVi
= 0 for all i, then s = 0.
Definition 4. Let F be a presheaf on X. The stalk of F at a point x X, denoted Fx, is the group whose
elements are equivalence classes of pairs
