 
Summary: Applications of Homological Algebra Introduction to Perverse Sheaves
Spring 2007 P. Achar
Operations on Sheaves; Adjointness Theorems
February 1, 2007
Definition 1. Let f : X Y be a continuous map of topological spaces. If F is a sheaf on X, the
pushforward by f of F, also called its direct image, and denoted fF, is the sheaf on Y defined by
(fF)(U) = F(f1
(U)).
Remark 2. Note that specifying a sheaf on a onepoint topological space is the same as specifying a single
abelian group. Onepoint spaces occur in several examples below, and we will frequently treat abelian groups
as sheaves on these spaces without further comment.
Example 3. Let f : X {x} be the constant map to a onepoint space. Then fF (X, F) for any
sheaf F on X.
Example 4. Let x0 X, and let i : {x0} X be the inclusion of the point. Let A be an abelian group,
thought of as a sheaf on {x0}. Then iA is sheaf on X whose stalks are all 0 except at x0, where its stalk is
isomorphic to A. This kind of sheaf is called a skyscraper sheaf.
Definition 5. Let F and G be sheaves on X. Their direct sum is the sheaf F G defined by (F G)(U) =
F(U) G(U) for all open sets U X.
Example 6. Let X = C {0}, and let f : X X be the map f(z) = z2
. Then fC C Q, where Q is
