Hannah Turner (Stockton University)

Hannah Turner (Stockton University)

Title: The (fractional) Dehn twist coefficient and infinite-type surfaces

Abstract:  The fractional Dehn twist coefficient (FDTC) is an invariant of a self-map of a surface which is some measure of how the map twists near a boundary component of the surface. It has mostly been studied for compact surfaces; in this setting the invariant is always a fraction. I will discuss work to give a new definition of the invariant which has a natural extension to infinite-type surfaces and show that it has surprising properties in this setting. In particular, the invariant no longer needs to be a fraction - any real number amount of twisting can be achieved! I will also discuss a new set of examples of (tame) big mapping classes called wagon wheel maps which exhibit irrational twisting behavior. This is joint work in progress with Diana Hubbard and Peter Feller.