# Dehn twist

A positive Dehn twist applied to a cylinder about the red curve c modifies the green curve as shown.

In geometric topology, a branch of mathematics, a Dehn twist is a certain type of self-homeomorphism of a surface (two-dimensional manifold).

## Definition

General Dehn twist on a compact surface represented by a n-gon.

Suppose that c is a simple closed curve in a closed, orientable surface S. Let A be a tubular neighborhood of c. Then A is an annulus, homeomorphic to the Cartesian product of a circle and a unit interval I:

${\displaystyle c\,\subset \,A\,\cong \,S^{1}\times I.}$

Give A coordinates (s, t) where s is a complex number of the form ${\displaystyle e^{i\theta }}$ with ${\displaystyle \theta \in [0,2\pi ],}$ and t ∈ [0,1].

Let f be the map from S to itself which is the identity outside of A and inside A we have

${\displaystyle \displaystyle f(s,t)=(se^{{i}2\pi t},t).}$

Then f is a Dehn twist about the curve c.

Dehn twists can also be defined on a non-orientable surface S, provided one starts with a 2-sided simple closed curve c on S.

## Example

An example of a Dehn twist on the torus, along the closed curve a, in blue, where a is an edge of the fundamental polygon representing the torus
The automorphism on the fundamental group of the torus induced by the self-homeomorphism of the Dehn twist along one of the generators of the torus

Consider the torus represented by a fundamental polygon with edges a and b

${\displaystyle \mathbb {T} ^{2}\cong \mathbb {R} ^{2}/\mathbb {Z} ^{2}.}$

Let a closed curve be the line along the edge a called ${\displaystyle \gamma _{a}}$.

Given the choice of gluing homeomorphism in the figure, a tubular neighborhood of the curve ${\displaystyle \gamma _{a}}$ will look like a band linked around a doughnut. This neighborhood is homeomorphic to an annulus, say

${\displaystyle a(0;0,1)=\{z\in \mathbb {C} :0<|z|<1\}}$

in the complex plane.

By extending to the torus the twisting map ${\displaystyle (e^{i\theta },t)\mapsto (e^{i(\theta +2\pi t)},t)}$ of the annulus, through the homeomorphisms of the annulus to an open cylinder to the neighborhood of ${\displaystyle \gamma _{a}}$, yields a Dehn twist of the torus by a.

${\displaystyle T_{a}:\mathbb {T} ^{2}\to \mathbb {T} ^{2}}$

This self homeomorphism acts on the closed curve along b. In the tubular neighborhood it takes the curve of b once along the curve of a.

A homeomorphism between topological spaces induces a natural isomorphism between their fundamental groups. Therefore one has an automorphism

${\displaystyle {T_{a}}_{\ast }:\pi _{1}(\mathbb {T} ^{2})\to \pi _{1}(\mathbb {T} ^{2}):[x]\mapsto [T_{a}(x)]}$

where [x] are the homotopy classes of the closed curve x in the torus. Notice ${\displaystyle {T_{a}}_{\ast }([a])=[a]}$ and ${\displaystyle {T_{a}}_{\ast }([b])=[b*a]}$, where ${\displaystyle b*a}$ is the path travelled around b then a.

## Mapping class group

The 3g − 1 curves from the twist theorem, shown here for g = 3.

It is a theorem of Max Dehn that maps of this form generate the mapping class group of isotopy classes of orientation-preserving homeomorphisms of any closed, oriented genus-${\displaystyle g}$ surface. W. B. R. Lickorish later rediscovered this result with a simpler proof and in addition showed that Dehn twists along ${\displaystyle 3g-1}$ explicit curves generate the mapping class group (this is called by the punning name "Lickorish twist theorem"); this number was later improved by Stephen P. Humphries to ${\displaystyle 2g+1}$, for ${\displaystyle g>1}$, which he showed was the minimal number.

Lickorish also obtained an analogous result for non-orientable surfaces, which require not only Dehn twists, but also "Y-homeomorphisms."