Note New Lunch Time: We shall be taking the speakers of the joint colloquium and the PDE seminar to lunch, departing from the outside foyer, level 2 Carslaw at 12:30 p.m. Stephan Tillmann (University of Melbourne) Title: What is the Thurston norm? Abstract: In the late seventies, Bill Thurston defined a semi-norm on the homology of a 3-dimensional manifold which lends itself to the study of manifolds which fibre over the circle. This led him to formulate the Virtual Fibration Conjecture, which is fairly inscrutable and implies almost all major results and conjectures in the field. Nevertheless, Thurston gave the conjecture "a definite chance for a positive answer" and much research is currently devoted to it. I will describe the Thurston norm, its main properties and applications, as well as its relationship to McMullen’s Alexander norm and the geometric invariant for groups due to Bieri, Neumann and Strebel.