Mapping class group

From Academic Kids

In mathematics, in the sub-field of geometric topology, the mapping class group is an important algebraic invariant of a topological space. Briefly, the mapping class group is a discrete group of 'symmetries' of the space.

To be more precise, suppose that X is a topological space. Let

<math>{\rm Homeo}(X)<math>

be the homeomorphism group of X. Let

<math>{\rm Homeo}_0(X)<math>

be the subgroup of <math>{\rm Homeo}(X)<math> consisting of all homeomorphisms isotopic to the identity map on X. It is easy to verify that <math>{\rm Homeo}_0(X)<math> is in fact a subgroup and is normal. The factor group

<math>{\rm MCG}(X) = {\rm Homeo}(X) / {\rm Homeo}_0(X)<math>

is then the mapping class group of X. Thus there is a natural short exact sequence:

<math>1 \rightarrow {\rm Homeo}_0(X) \rightarrow {\rm Homeo}(X) \rightarrow {\rm MCG}(X) \rightarrow 1<math>

As usual, there is interest in the spaces where this sequence splits. If the mapping class group of X is finite then X is sometimes called rigid.

Some mathematicians, when X is an orientable manifold, restrict attention to orientation-preserving homeomorphisms <math>{\rm Homeo}^+(X)<math>. Here convention dictates that the group defined in the second paragraph be called the extended mapping class group.

Examples

An easy exercise is to show that:

<math> {\rm MCG}(S^1) = {\mathbb Z}/2{\mathbb Z}. <math>

For manifolds of dimension two or higher the mapping class group is often infinite. Generalizing the above, for the n-torus we have:

<math> {\rm MCG}(T^n) = {\rm GL}(n, {\mathbb Z}). <math>

Finally, the mapping class groups of surfaces have been heavily studied. (Note the special case of <math> {\rm MCG}(T^2)<math> above.) This is perhaps due to their strange similarity to higher rank linear groups as well as many applications, via surface bundles, in Thurston's theory of geometric three-manifolds. The mapping class group of any closed, orientable surface can be generated by Dehn twists.

Another fine example is the mapping class group of the Klein bottle <math> K <math>. This is

<math> {\rm MCG}(K)={\mathbb Z}/2{\mathbb Z}+{\mathbb Z}/2{\mathbb Z}. <math>

Determined topologically by W.B.R. Lickorish in 1963 and algebraically by H. Torriani in 1987.

References

For more information on this last topic consult the book by Andrew Casson and Steve Bleiler entitled Automorphisms of surfaces after Nielsen and Thurston. The special case where X is a punctured disk is discussed by Joan Birman in Braids, Links, and Mapping Class Groups.

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