In

Algebraic Topology

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algebraic topology
Algebraic topology is a branch of mathematics that uses tools from abstract algebra to study topological spaces. The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify ...

the cap product is a method of adjoining a chain of degree ''p'' with a cochain
In mathematics, a chain complex is an algebraic structure that consists of a sequence of abelian groups (or modules) and a sequence of homomorphisms between consecutive groups such that the image of each homomorphism is included in the kernel of th ...

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
Hassler Whitney (March 23, 1907 – May 10, 1989) was an American mathematician. He was one of the founders of singularity theory, and did foundational work in manifolds, embeddings, immersions, characteristic classes, and geometric integratio ...

in 1938.
Definition

Let ''X'' be atopological space
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called po ...

and ''R'' a coefficient ring. The cap product is a bilinear map
In mathematics, a bilinear map is a function combining elements of two vector spaces to yield an element of a third vector space, and is linear in each of its arguments. Matrix multiplication is an example.
Definition
Vector spaces
Let V, W ...

on singular homology
In algebraic topology, singular homology refers to the study of a certain set of algebraic invariants of a topological space ''X'', the so-called homology groups H_n(X). Intuitively, singular homology counts, for each dimension ''n'', the ''n''- ...

and cohomology
In mathematics, specifically in homology theory and algebraic topology, cohomology is a general term for a sequence of abelian groups, usually one associated with a topological space, often defined from a cochain complex. Cohomology can be viewe ...

:$\backslash frown\backslash ;:\; H\_p(X;R)\backslash times\; H^q(X;R)\; \backslash rightarrow\; H\_(X;R).$
defined by contracting a singular chain
In algebraic topology, singular homology refers to the study of a certain set of algebraic invariants of a topological space ''X'', the so-called homology groups H_n(X). Intuitively, singular homology counts, for each dimension ''n'', the ''n''-d ...

$\backslash sigma\; :\; \backslash Delta\backslash \; ^p\; \backslash rightarrow\backslash \; X$ with a singular cochain
In mathematics, a chain complex is an algebraic structure that consists of a sequence of abelian groups (or modules) and a sequence of homomorphisms between consecutive groups such that the image of each homomorphism is included in the kernel of th ...

$\backslash psi\; \backslash in\; C^q(X;R),$ by the formula :
:$\backslash sigma\; \backslash frown\; \backslash psi\; =\; \backslash psi(\backslash sigma,\; \_)\; \backslash sigma,\; \_.$
Here, the notation $\backslash sigma,\; \_$ indicates the restriction of the simplicial map $\backslash sigma$ 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 theKünneth formula Künneth is a surname. Notable people with the surname include:
* Hermann Künneth (1892–1975), German mathematician
* Walter Künneth (1901–1997), German Protestant theologian
{{DEFAULTSORT:Kunneth
German-language surnames ...

, we can explain the existence of the cap product in the following way. Using CW approximation we may assume that $X$ is a CW-complex and $C\_\backslash bullet(X)$ (and $C^\backslash bullet(X)$) is the complex of its cellular chains (or cochains, respectively). Consider then the composition
$$C\_\backslash bullet(X)\; \backslash otimes\; C^\backslash bullet(X)\; \backslash overset\; C\_\backslash bullet(X)\; \backslash otimes\; C\_\backslash bullet(X)\; \backslash otimes\; C^\backslash bullet(X)\; \backslash overset\; C\_\backslash bullet(X)$$
where we are taking tensor products of chain complexes, $\backslash Delta\; \backslash colon\; X\; \backslash to\; X\; \backslash times\; X$ is the diagonal map which induces the map
$$\backslash Delta\_*\; \backslash colon\; C\_\backslash bullet(X)\backslash to\; C\_\backslash bullet(X\; \backslash times\; X)\backslash cong\; C\_\backslash bullet(X)\backslash otimes\; C\_\backslash bullet(X)$$
on the chain complex, and $\backslash varepsilon\; \backslash colon\; C\_p(X)\; \backslash otimes\; C^q(X)\; \backslash to\; \backslash mathbb$ is the evaluation map (always 0 except for $p=q$).
This composition then passes to the quotient to define the cap product $\backslash frown\; \backslash colon\; H\_\backslash bullet(X)\; \backslash times\; H^\backslash bullet(X)\; \backslash to\; H\_\backslash bullet(X)$, and looking carefully at the above composition shows that it indeed takes the form of maps $\backslash frown\; \backslash colon\; H\_p(X)\; \backslash times\; H^q(X)\; \backslash to\; H\_(X)$, which is always zero for $p\; <\; q$.
Relation with Poincaré duality

For a closed orientable n-manifold M, we can define its fundamental class $;\; href="/html/ALL/l/.html"\; ;"title="">$The slant product

If in the above discussion one replaces $X\backslash times\; X$ by $X\backslash times\; Y$, the construction can be (partially) replicated starting from the mappings $$C\_\backslash bullet(X\backslash times\; Y)\; \backslash otimes\; C^\backslash bullet(Y)\backslash cong\; C\_\backslash bullet(X)\; \backslash otimes\; C\_\backslash bullet(Y)\; \backslash otimes\; C^\backslash bullet(Y)\; \backslash overset\; C\_\backslash bullet(X)$$ and $$C^\backslash bullet(X\backslash times\; Y)\; \backslash otimes\; C\_\backslash bullet(Y)\backslash cong\; C^\backslash bullet(X)\; \backslash otimes\; C^\backslash bullet(Y)\; \backslash otimes\; C\_\backslash bullet(Y)\; \backslash overset\; C^\backslash bullet(X)$$ to get, respectively, slant products $/$: $$H\_p(X\backslash times\; Y;R)\; \backslash otimes\; H^q(Y;R)\; \backslash rightarrow\; H\_(X;R)$$ and $$H^p(X\backslash times\; Y;R)\; \backslash otimes\; H\_q(Y;R)\; \backslash rightarrow\; H^(X;R).$$ In case ''X = Y'', the first one is related to the cap product by the diagonal map: $\backslash Delta\_*(a)/\backslash phi\; =\; a\backslash frown\; \backslash phi$. 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 : :$\backslash partial(\backslash sigma\; \backslash frown\; \backslash psi)\; =\; (-1)^q(\backslash partial\; \backslash sigma\; \backslash frown\; \backslash psi\; -\; \backslash sigma\; \backslash frown\; \backslash delta\; \backslash psi).$ Given a map ''f'' the induced maps satisfy : :$f\_*(\; \backslash sigma\; )\; \backslash frown\; \backslash psi\; =\; f\_*(\backslash sigma\; \backslash frown\; f^*\; (\backslash psi)).$ The cap and cup product are related by : :$\backslash psi(\backslash sigma\; \backslash frown\; \backslash varphi)\; =\; (\backslash varphi\; \backslash smile\; \backslash psi)(\backslash sigma)$ where :$\backslash sigma\; :\; \backslash Delta\; ^\; \backslash rightarrow\; X$, $\backslash psi\; \backslash in\; C^q(X;R)$ and $\backslash varphi\; \backslash in\; C^p(X;R).$ An interesting consequence of the last equation is that it makes $H\_(X;R)$ into a right $H^(X;R)-$ module.See also

* Cup product *Poincaré duality
In mathematics, the Poincaré duality theorem, named after Henri Poincaré, is a basic result on the structure of the homology and cohomology groups of manifolds. It states that if ''M'' is an ''n''-dimensional oriented closed manifold (compact ...

*Singular homology
In algebraic topology, singular homology refers to the study of a certain set of algebraic invariants of a topological space ''X'', the so-called homology groups H_n(X). Intuitively, singular homology counts, for each dimension ''n'', the ''n''- ...

* Homology theory
References

* Hatcher, A.,Algebraic Topology

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Cambridge University Press
Cambridge University Press is the university press of the University of Cambridge. Granted letters patent by King Henry VIII in 1534, it is the oldest university press in the world. It is also the King's Printer.
Cambridge University Pre ...

(2002) . Detailed discussion of homology theories for simplicial complexes and manifolds, singular homology, etc.
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{{DEFAULTSORT:Cap Product
Homology theory
Algebraic topology
Binary operations