Intersection norms on surfaces and Birkhoff surfaces for geodesic flows

Pierre Dehornoy

Published 2016-04-22Version 1

For every finite collection of curves on a surface, we define an associated (semi-)norm on the first homology group of the surface. The unit ball of the dual norm is the convex hull of finitely many integer points. We give an interpretation of these points in terms of certain coorien-tations of the original collection of curves. Our main result is a classification statement: when the surface has constant curvature and the curves are geodesics, integer points in the interior of the unit ball of the dual norm classify isotopy classes of Birkhoff sections for the geodesic flow (on the unit tangent bundle to the surface) whose boundary is the symmetric lift of the collection of geodesics. These Birkhoff sections also yield numerous open-book decompositions of the unit tangent bundle.