# Verdier duality

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In mathematics, **Verdier duality** is a duality in sheaf theory that generalizes Poincaré duality for manifolds. Verdier duality was introduced by Template:Harvs as an analog for locally compact spaces of the coherent duality for schemes due to Grothendieck. It is commonly encountered when studying constructible or perverse sheaves.

## Verdier duality

Verdier duality states that certain image functors for sheaves are actually adjoint functors. There are two versions.

**Global Verdier duality** states that for a continuous map , the derived functor of the direct image with proper supports *Rf*_{!} has a right adjoint *f*^{!} in the derived category of sheaves, in other words, for a sheaf on X and on Y we have

The exclamation mark is often pronounced "shriek" (slang for exclamation mark), and the maps called "*f* shriek" or "*f* lower shriek" and "*f* upper shriek" – see also shriek map.

**Local Verdier duality** states that

in the derived category of sheaves of *k* modules over *X*.
It is important to note that the distinction between the global and local versions is that the former relates maps between sheaves, whereas the latter relates (complexes of) sheaves directly and so can be evaluated locally. Taking global sections of both sides in the local statement gives global Verdier duality.

The **dualizing complex** *D _{X}* on

*X*is defined to be

where *p* is the map from *X* to a point. Part of what makes Verdier duality interesting in the singular setting is that when *X* is not a manifold (a graph or singular algebraic variety for example) then the dualizing complex is not quasi-isomorphic to a sheaf concentrated in a single degree. From this perspective the derived category is necessary in the study of singular spaces.

If *X* is a finite-dimensional locally compact space, and *D*^{b}(*X*) the bounded derived category of sheaves of abelian groups over *X*, then the **Verdier dual** is a contravariant functor

defined by

It has the following properties:

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## Poincaré duality

Poincaré duality can be derived as a special case of Verdier duality. Here one explicitly calculates cohomology of a space using the machinery of sheaf cohomology.

Suppose *X* is a compact *n*-dimensional manifold, *k* is a field and *k*_{X} is the constant sheaf on *X* with coefficients in *k*. Let *f=p* be the constant map. Global Verdier duality then states

To understand how Poincaré duality is obtained from this statement, it is perhaps easiest to understand both sides piece by piece. Let

be an injective resolution of the constant sheaf. Then by standard facts on right derived functors

is a complex whose cohomology is the compactly supported cohomology of *X*. Since morphisms between complexes of sheaves (or vector spaces) themselves form a complex we find that

where the last non-zero term is in degree 0 and the ones to the left are in negative degree. Morphisms in the derived category are obtained from the homotopy category of chain complexes of sheaves by taking the zeroth cohomology of the complex, i.e.

For the other side of the Verdier duality statement above, we have to take for granted the fact that when *X* is a compact *n*-dimensional manifold

which is the dualizing complex for a manifold. Now we can re-express the right hand side as

We finally have obtained the statement that

By repeating this argument with the sheaf *k*_{X} replaced with the same sheaf placed in degree *i* we get the classical Poincaré duality

## See also

## References

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