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In algebraic topology the **cap product** is a method of adjoining a chain of degree *p* with a cochain of degree *q*, such that *q* ≤ *p*, to form a composite chain of degree *p* − *q*. It was introduced by Eduard Čech in 1936, and independently by Hassler Whitney in 1938.

Let *X* be a topological space and *R* a coefficient ring. The cap product is a bilinear map on singular homology and cohomology

defined by contracting a singular chain with a singular cochain by the formula:

Here, the notation indicates the restriction of the simplicial map to its face spanned by the vectors of the base, see Simplex.

In analogy with the interpretation of the cup product in terms of the Künneth formula, we can explain the existence of the cap product in the following way. Using CW approximation we may assume that is a CW-complex and (and ) is the complex of its cellular chains (or cochains, respectively). Consider then the composition

where we are taking tensor products of chain complexes, is the diagonal map which induces the map

on the chain complex, and is the evaluation map (always 0 except for ).

This composition then passes to the quotient to define the cap product , and looking carefully at the above composition shows that it indeed takes the form of maps , which is always zero for .

For any point in , we have the long-exact sequence in homology (with coefficients in ) of the pair (M, M - {x}) (See Relative homology)

An element of is called the fundamental class for if is a generator of . A fundamental class of exists if is closed and R-orientable. In fact, if is a closed, connected and -orientable manifold, the map is an isomorphism for all in and hence, we can choose any generator of as the fundamental class.

For a closed -orientable n-manifold with fundamental class in (which we can choose to be any generator of ), the cap product map

is an isomorphism for all . This result is famously called Poincaré duality.

If in the above discussion one replaces by , the construction can be (partially) replicated starting from the mappings

and

to get, respectively, **slant products** :

and

In case *X = Y*, the first one is related to the cap product by the diagonal map: .

These ‘products’ are in some ways more like division than multiplication, which is reflected in their notation.

The boundary of a cap product is given by :

Given a map *f* the induced maps satisfy :

The cap and cup product are related by :

where

- , and

If is allowed to be of higher degree than , the last identity takes a more general form

which makes into a right -module.