Dehn twists on nonorientable surfaces
Volume 189 / 2006
Abstract
Let be the Dehn twist about a circle a on an orientable surface. It is well known that for each circle b and an integer n, I(t_a^n(b),b)=|n|I(a,b)^2, where I(\cdot,\cdot) is the geometric intersection number. We prove a similar formula for circles on nonorientable surfaces. As a corollary we prove some algebraic properties of twists on nonorientable surfaces. We also prove that if {\cal M}(N) is the mapping class group of a nonorientable surface N, then up to a finite number of exceptions, the centraliser of the subgroup of {\cal M}(N) generated by the twists is equal to the centre of {\cal M}(N) and is generated by twists about circles isotopic to boundary components of N.