The Mahler lectures are a biennial activity organised by the Australian Mathematical Society. In 2009 we have partnered with the Clay Mathematical Institute to combine the Mahler Lectures and the Clay Lectures into the 2009 Clay–Mahler Lecture Tour, with funding also from the Australian Mathematical Sciences Institute.

• 23rd July, Algebra/Geometry/Topology Seminar in Russell Love Theatre, Richard Berry Building, Melbourne University, 14:15–16:15;
  Danny Calegari: Surface subgroups from homology
  

For more information: contact Arun Ram (A.Ram@ms.unimelb.edu.au)
or email: craigw@ms.unimelb.edu.au

Abstract: Surface subgroups from homology

This is the first lecture in a series on scl (stable commutator length). scl answers the question: what is the simplest surface in a given space with prescribed boundary?, where simplest is interpreted in topological terms. This topological definition is complemented by several equivalent definitions:

• in group theory, as a measure of non-commutativity of a group; and
• in linear programming, as the solution of a certain linear optimization problem.

On the topological side, scl is concerned with questions such as computing the genus of a knot, or finding the simplest 4-manifold that bounds a given 3-manifold. On the linear programming side, scl is measured in terms of certain functions called quasimorphisms, which arise from hyperbolic geometry (negative curvature) and symplectic geometry (causal structures).

We will discuss how scl in free and surface groups is connected to such diverse phenomena as the existence of closed surface subgroups in graphs of groups, rigidity and discreteness of symplectic representations, bounding immersed curves on a surface by immersed subsurfaces, and the theory of multi-dimensional continued fractions and Klein polyhedra.