Editing Excision theorem

{{Short description|Theorem in algebraic topology}}

In [[algebraic topology]], a branch of [[mathematics]], the '''excision theorem''' is a theorem about [[relative homology]] and one of the [[Eilenberg–Steenrod axioms]]. Given a topological space <math>X</math> and subspaces <math>A</math> and <math>U</math> such that <math>U</math> is also a subspace of <math>A</math>, the theorem says that under certain circumstances, we can cut out ('''excise''') <math>U</math> from both spaces such that the [[Relative_homology|relative homologies]] of the pairs <math>(X \setminus U,A \setminus U )</math> into <math>(X, A)</math> are isomorphic.

This assists in computation of [[singular homology]] groups, as sometimes after excising an appropriately chosen subspace we obtain something easier to compute.
== Theorem ==

=== Statement === 
If <math>U\subseteq A \subseteq X</math> are as above, we say that <math>U</math> can be '''excised''' if the inclusion map of the pair <math>(X \setminus U,A \setminus U )</math> into <math>(X, A)</math> induces an isomorphism on the relative homologies: 

{{center|<math>H_n(X \setminus U,A \setminus U) \cong H_n(X,A)</math>}}

The theorem states that if the [[Closure (topology)|closure]]  of <math>U</math> is contained in the [[Interior (topology)|interior]] of <math>A</math>, then <math>U</math> can be excised.

Often, subspaces that do not satisfy this containment criterion still can be excised&mdash;it suffices to be able to find a [[deformation retract]] of the subspaces onto subspaces that do satisfy it.

=== Proof sketch ===
The proof of the excision theorem is quite intuitive, though the details are rather involved. The idea is to subdivide the simplices in a relative cycle in <math>(X, A)</math> to get another chain consisting of "smaller" simplices (this can be done using [[barycentric subdivision]]<ref>See Hatcher 2002, p.119</ref>), and continuing the process until each simplex in the chain lies entirely in the interior of <math>A</math> or the interior of  <math>X \setminus U</math>. Since these form an open cover for  <math>X</math> and simplices are [[Compact space|compact]], we can eventually do this in a finite number of steps. This process leaves the original homology class of the chain unchanged (this says the subdivision operator is [[chain homotopy|chain homotopic]] to the identity map on homology).
In the relative homology  <math>H_n(X, A)</math>, then, this says all the terms contained entirely in the interior of <math>U</math> can be dropped without affecting the homology class of the cycle. This allows us to show that the inclusion map is an isomorphism, as each relative cycle is  equivalent to one that avoids <math>U</math> entirely.

== Applications ==

=== Eilenberg–Steenrod axioms ===

The excision theorem is taken to be one of the [[Eilenberg–Steenrod axioms]].

=== Mayer–Vietoris sequences ===

The [[Mayer–Vietoris sequence]] may be derived with a combination of excision theorem and the long-exact sequence.<ref>See Hatcher 2002, p.149, for example</ref>

=== Suspension theorem for homology ===

The excision theorem may be used to derive the suspension theorem for homology,  which says <math>\tilde{H}_n(X) \cong \tilde{H}_{n+1}(SX)</math> for all <math>n</math>, where <math>SX</math> is the [[Suspension (topology)|suspension]] of <math>X</math>.<ref>See Hatcher 2002, p.132, for example</ref>

=== Invariance of dimension===
If nonempty open sets <math> U\subset \mathbb{R}^n</math> and <math> V\subset \mathbb{R}^m</math>
are homeomorphic, then ''m'' = ''n''. This follows from the excision theorem, the long exact sequence for the pair <math>(\mathbb{R}^n,\mathbb{R}^n-x)</math>, and the fact that <math> \mathbb{R}^n-x</math> deformation retracts onto a sphere.
In particular, <math>\mathbb{R}^n</math> is not homeomorphic to <math>\mathbb{R}^m</math> if <math>m\neq n</math>.<ref>See Hatcher 2002, p.135</ref>
== See also ==
*[[Homotopy excision theorem]]

==References==
{{reflist}}

==Bibliography==
* [[Joseph J. Rotman]], ''An Introduction to Algebraic Topology'', Springer-Verlag, {{ISBN|0-387-96678-1}}
* [[Allen Hatcher]], [http://pi.math.cornell.edu/~hatcher/AT/ATpage.html ''Algebraic Topology.''] Cambridge University Press, Cambridge, 2002.

[[Category:Homology theory]]
[[Category:Theorems in topology]]