The City College of New YorkCCNY
Department of Mathematics
Division of Science

The rotation number for maps on graphs

Mathematics Colloquium

Time and place

1 PM on Thursday, November 21st, 2013; NAC 6113

Christopher Staecker (Fairfield University)

Abstract

While studying the precession of planetary orbits, Poincare introduced the rotation number for a homeomorphism of a circle. This is the "average" angle that points traverse under the homeomorphism, and has become a fundamental tool in the theory of dynamics on the circle. The basic concept has been extended to maps on the annulus and torus, and some other settings.

We will discuss a generalization of the rotation number to maps of finite graphs which has been developed in joint work with Chris Bernhardt. Using the fundamental group and some basic covering space theory we describe a rotation element which obeys natural properties analogous to the classical rotation number. This invariant naturally takes the form of an element of a "free group with noninteger exponents". We will discuss recent results as well as some new work in progress.

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