We have spent a while in the past few days going through the rather categorical formalism of the upper shriek functor f^! obtained from a map f: X \to Y between locally compact Hausdorff spaces of finite cohomological dimension. That is, we showed that the upper shriek must exist on the derived category; this was Verdier duality. However, so far we have not seen any concrete applications of this formalism. I actually feel a bit guilty about having not indicated better some of these in the introductory post and having essentially plunged into the abstract nonsense.

Now we shall apply the existence of {f^!} to questions involving manifolds. Once we know that {f^!} exists, we will be able to describe it using the adjoint property rather simply (for manifolds). This will lead to clean statements of theorems in algebraic topology. For instance, Poincaré duality will be a direct consequence of the fact that, on an {n}-dimensional oriented manifold, the dualizing sheaf (see below) is just {k[n]}.

1. The dualizing complex

After wading through the details of the proof of Verdier duality, let us now consider the simpler case where {Y = \left\{\ast\right\}}. {X} is still a locally compact space of finite dimension, and {k} remains a noetherian ring. Then Verdier duality gives a right adjoint {f^!} to the functor {\mathbf{R} \Gamma_c: \mathbf{D}^+(X, k) \rightarrow \mathbf{D}^+(k)}. In other words, for each {\mathcal{F}^\bullet \in \mathbf{D}^+(X, k)} and each complex {G^\bullet} of {k}-modules, we have an isomorphism

\displaystyle \hom_{\mathbf{D}^+(k)}(\mathbf{R} \Gamma_c (\mathcal{F}^\bullet), G^\bullet) \simeq \hom_{\mathbf{D}^+(X, k)}(\mathcal{F}^\bullet, f^!(G^\bullet)).

Of course, the category {\mathbf{D}^+(k)} is likely to be much simpler than {\mathbf{D}^+(X, k)}, especially if, say, {k} is a field.

Definition 1 {\mathcal{D}^\bullet = f^!(k)} is called the dualizing complex on the space {X}. {\mathcal{D}^\bullet} is an element of the derived category {\mathbf{D}^+(X, k)}, and is well-defined there. We will always assume that {\mathcal{D}^\bullet} is a bounded-below complex of injective sheaves. (more…)