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 by considering the composition
in terms of the chain and cochain complexes of , 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 .
The slant product
The above discussion indicates that the same operation can be defined on cartesian products yielding a product
In case X = Y, the two products are related by the diagonal map.
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 :
- , and
An interesting consequence of the last equation is that it makes into a right module.