SIMPLIFYING INDEFINITE FIBRATIONS ON 4-MANIFOLDS

The main goal of this article is to connect some recent perspectives in the study of 4-manifolds from the vantage point of singularity theory. We present explicit algorithms for simplifying the topology of various maps on 4-manifolds, which include broken Lefschetz fibrations and indefinite Morse 2-functions. The algorithms consist of sequences of moves, which modify indefinite fibrations in smooth 1-parameter families. These algorithms allow us to give purely topological and constructive proofs of the existence of simplified broken Lefschetz fibrations and Morse 2-functions on general 4-manifolds, and a theorem of Auroux–Donaldson–Katzarkov on the existence of certain broken Lefschetz pencils on near-symplectic 4-manifolds. We moreover establish a correspondence between broken Lefschetz fibrations and Gay–Kirby trisections of 4-manifolds, and show the existence and stable uniqueness of simplified trisections on all 4-manifolds. Building on this correspondence, we also provide several new constructions of trisections, including infinite families of genus-3 trisections with homotopy inequivalent total spaces, and exotic same genera trisections of 4-manifolds in the homeomorphism classes of complex rational surfaces.