Cap product


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.

Definition

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.

Interpretation

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 is the complex of its cellular chains. 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.
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

If in the above discussion one replaces by, the construction can be replicated starting from the mappings
to get, respectively, slant products :
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.

Equations

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
An interesting consequence of the last equation is that it makes into a right module.