# Mapping class group

In mathematics, in the subfield of geometric topology, the **mapping class group** is an important algebraic invariant of a topological space. Briefly, the mapping class group is a certain discrete group corresponding to symmetries of the space.

Consider a topological space, that is, a space with some notion of closeness between points in the space. We can consider the set of homeomorphisms from the space into itself, that is, continuous maps with continuous inverses: functions which stretch and deform the space continuously without breaking or glueing the space. This set of homeomorphisms can be thought of as a space itself. It forms a group under functional composition. We can also define a topology on this new space of homeomorphisms. The open sets of this new function space will be made up of sets of functions that map compact subsets *K* into open subsets *U* as *K* and *U* range throughout our original topological space, completed with their finite intersections (which must be open by definition of topology) and arbitrary unions (again which must be open). This gives a notion of continuity on the space of functions, so that we can consider continuous deformation of the homeomorphisms themselves: called homotopies. We define the mapping class group by taking homotopy classes of homeomorphisms, and inducing the group structure from the functional composition group structure already present on the space of homeomorphisms.

If working in the homotopy category, the mapping class group of *X* is the group of homotopy classes of homotopy equivalences of *X*.

The four elements are the identity, a Dehn twist on a two-sided curve which does not bound a Möbius strip, the y-homeomorphism of Lickorish, and the product of the twist and the y-homeomorphism. It is a nice exercise to show that the square of the Dehn twist is isotopic to the identity.

We also remark that the closed genus three non-orientable surface *N*_{3} (the connected sum of three projective planes) has:

Mapping class groups of 3-manifolds have received considerable study as well, and are closely related to mapping class groups of 2-manifolds. For example, any finite group can be realized as the mapping class group (and also the isometry group) of a compact hyperbolic 3-manifold.^{[6]}

Given a pair of spaces *(X,A)* the mapping class group of the pair is the isotopy-classes of automorphisms of the pair, where an automorphism of *(X,A)* is defined as an automorphism of *X* that preserves *A*, i.e. *f*: *X* → *X* is invertible and *f(A)* = *A*.

If *K* ⊂ **S**^{3} is a knot or a link, the **symmetry group of the knot (resp. link)** is defined to be the mapping class group of the pair (**S**^{3}, *K*). The symmetry group of a hyperbolic knot is known to be dihedral or cyclic, moreover every dihedral and cyclic group can be realized as symmetry groups of knots. The symmetry group of a torus knot is known to be of order two **Z**_{2}.

Notice that there is an induced action of the mapping class group on the homology (and cohomology) of the space *X*. This is because (co)homology is functorial and Homeo_{0} acts trivially (because all elements are isotopic, hence homotopic to the identity, which acts trivially, and action on (co)homology is invariant under homotopy). The kernel of this action is the *Torelli group*, named after the Torelli theorem.

In the case of orientable surfaces, this is the action on first cohomology *H*^{1}(Σ) ≅ **Z**^{2g}. Orientation-preserving maps are precisely those that act trivially on top cohomology *H*^{2}(Σ) ≅ **Z**. *H*^{1}(Σ) has a symplectic structure, coming from the cup product; since these maps are automorphisms, and maps preserve the cup product, the mapping class group acts as symplectic automorphisms, and indeed all symplectic automorphisms are realized, yielding the short exact sequence:

The symplectic group is well understood. Hence understanding the algebraic structure of the mapping class group often reduces to questions about the Torelli group.

Note that for the torus (genus 1) the map to the symplectic group is an isomorphism, and the Torelli group vanishes.