Gauge Theory and Topology Seminar: The (fractional) Dehn twist coefficient and infinite-type surfaces

SEMINARS, GAUGE-TOPOLOGY-SYMPLECTIC

Speaker:

Hannah Turner - Georgia Institute of Technology

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 been studied for compact (or finite-type) surfaces; in this setting the invariant is always a fraction. I will discuss work to extend this invariant 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 with Diana Hubbard and Peter Feller.