Functions on surfaces and constructions of manifolds in dimensions three, four and five

Abstract

We offer a new proof that two closed oriented 4-manifolds are cobordant if their signatures agree, in the spirit of Lickorish's proof that all closed oriented 3-manifolds bound 4-manifolds. Where Lickorish uses Heegaard splittings we use trisections. In fact we begin with a subtle recasting of Lickorish's argument: Instead of factoring the gluing map for a Heegaard splitting as a product of Dehn twists, we encode each handlebody in a Heegaard splitting in terms of a Morse function on the surface and build the 4-manifold from a generic homotopy between the two functions. This extends up a dimension by encoding a trisection of a closed 4-manifold as a triangle (circle) of functions and constructing an associated 5-manifold from an extension to a 2-simplex (disk) of functions. This borrows ideas from Hatcher and Thurston's proof that the mapping class group of a surface is finitely presented.