Wrinkled fibrations on near-symplectic manifolds

Abstract

Motivated by the programmes initiated by Taubes and Perutz, we study the geometry of near-symplectic $4$–manifolds, ie, manifolds equipped with a closed $2$–form which is symplectic outside a union of embedded $1$–dimensional submanifolds, and broken Lefschetz fibrations on them; see Auroux, Donaldson and Katzarkov adk and Gay and Kirby gkirby. We present a set of four moves which allow us to pass from any given broken fibration to any other which is deformation equivalent to it. Moreover, we study the change of the near-symplectic geometry under each of these moves. The arguments rely on the introduction of a more general class of maps, which we call wrinkled fibrations and which allow us to rely on classical singularity theory. Finally, we illustrate these constructions by showing how one can merge components of the zero-set of the near-symplectic form. We also disprove a conjecture of Gay and Kirby by showing that any achiral broken Lefschetz fibration can be turned into a broken Lefschetz fibration by applying a sequence of our moves.