# Poincaré duality

In mathematics, the Poincaré duality theorem is a basic result on the structure of the homology and cohomology groups of manifolds. It states that if M is an n-dimensional compact oriented manifold, then the k-th cohomology group of M is isomorphic to the (nk)-th homology group of M, for all integers k. It further states that if mod 2 homology and cohomology is used, then the assumption of orientability can be dropped.

## History

A form of Poincaré duality was first stated, without proof, by Henri Poincaré in 1893. It was stated in terms of Betti numbers: The k-th and (nk)-th Betti numbers of a closed (i.e. compact and without boundary) orientable n-manifold are equal. The cohomology concept was at that time about 40 years from being clarified. In his 1895 paper Analysis Situs, Poincaré tried to prove the theorem using topological intersection theory, which he had invented. Criticism of his work by Poul Heegaard led him to realize that his proof was seriously flawed. In the first two complements to Analysis Situs, Poincaré gave a new proof in terms of dual triangulations.

Poincaré duality did not take on its modern form until the advent of cohomology in the 1930's, when Eduard Cech and Hassler Whitney invented the cup and cap products and formulated Poincaré duality in these new terms.

## Dual cell structures

Poincaré duality was classically thought of in terms of dual triangulations, which are generalizations of dual polyhedra. Given a triangulation X of an n-dimensional manifold M, one replaces each k-simplex with a (nk)-cell to produce a new decomposition Y of M. If each (nk)-cell is indeed a simplex then one says that Y is the dual triangulation of X. Considering the tetrahedron as a triangulation of the 2-sphere, the dual triangulation of the tetrahedron is another tetrahedron. This construction does not necessarily yield another triangulation, as the examples of the octahedron and icosahedron demonstrate. Poincaré used a (not entirely correct) method involving barycentric subdivision to show that we may always obtain a dual triangulation for compact oriented manifolds.

In more precise terms, one may describe the dual of a triangulation X as a triangulation Y such that given a k-simplex α in X, there is one (nk)-simplex in Y whose intersection number with α is 1, and such that the intersection number of α with any other (nk)-simplex of Y is 0.

The boundary operator in a chain complex can be viewed as a matrix. Let M be a closed n-manifold, X a triangulation of M, and Y the dual triangulation of X. Then one can show that the boundary operator

$\displaystyle C_p(X) \to C_{p-1}(X)$

is the transpose of the boundary operator

$\displaystyle C_{n-p+1}(Y) \to C_{n-p}(Y)$

Using the fact that the homology groups of a manifold are independent of the triangulation used to compute them, one can easily show that the k-th and (nk)-th Betti numbers of M are equal.

## Modern formulation

The modern statement of the Poincaré duality theorem is in terms of homology and cohomology: if M is a closed oriented n-manifold, and k is an integer, then there is a canonically defined isomorphism from the k-th homology group Hk(M) to the (nk)-th cohomology group Hnk(M). (Here, homology and cohomology is taken with coefficients in the ring of integers, but the isomorphism holds for any coefficient ring.) Specifically, one maps an element of Hk(M) to its cap product with a fundamental class of M, which will exist for oriented M.

Homology and cohomology groups are defined to be zero for negative degrees, so Poincaré duality in particular implies that the homology and cohomology groups of orientable closed n-manifolds are zero for degrees bigger than n.

### Naturality

Note that Hk is a contravariant functor while Hnk is covariant. The family of isomorphisms

DM : Hk(M) → Hnk(M)

is natural in the following sense: if

f : MN

is a continuous map between two oriented n-manifolds which is compatible with orientation, i.e. which maps the fundamental class of M to the fundamental class of N, then

DN = f* DM f*,

where f* and f* are the maps induced by f in homology and cohomology, respectively.

## Generalizations and related results

The Poincaré-Lefschetz duality theorem is a generalisation for manifolds with boundary. In the non-orientable case, taking into account the sheaf of local orientations, one can give a statement that is independent of orientability.

With the development of homology theory to include K-theory and other extraordinary theories from about 1955, it was realised that the homology H* could be replaced by other theories, once the products on manifolds were constructed; and there are now textbook treatments in generality.

There are many other forms of geometric duality in algebraic topology, including Lefschetz duality, Alexander duality and S-duality (homotopy theory).