Lefschetz fibrations on nonorientable 4-manifolds

TL;DR

This paper extends Lefschetz fibration theory to nonorientable 4-manifolds, showing they admit such fibrations, and derives consequences for open book decompositions and trisection diagrams, including explicit constructions.

Contribution

It introduces nonorientable Lefschetz fibrations on 4-manifolds without 3- and 4-handles, and constructs explicit open book and trisection diagrams for nonorientable 4-manifolds.

Findings

Nonorientable 4-manifolds admit Lefschetz fibrations over the disk.

Every nonorientable closed 3-manifold admits an open book decomposition.

Constructed explicit minimal open books and trisection diagrams for specific nonorientable 4-manifolds.

Abstract

Let $W$ be a nonorientable $4$-dimensional handlebody without $3$- and $4$-handles. We show that $W$ admits a Lefschetz fibration over the $2$-disk, whose regular fiber is a nonorientable surface with nonempty boundary. This is an analogue of a result of Harer obtained in the orientable case. As a corollary, we obtain a $4$-dimensional proof of the fact that every nonorientable closed $3$-manifold admits an open book decomposition, which was first proved by Berstein and Edmonds using branched coverings. Moreover, the monodromy of the open book we obtain for a given $3$-manifold belongs to the twist subgroup of the mapping class group of the page. In particular, we construct an explicit minimal open book for the connected sum of arbitrarily many copies of the product of the circle with the real projective plane. We also obtain a relative trisection diagram for $W$, based on the nonorientable Lefschetz fibration we construct, similar to the orientable case first studied by Castro. As a corollary, we get trisection diagrams for some closed $4$-manifolds, e.g. the product of the $2$-sphere with the real projective plane, by doubling $W$. Moreover, if $X$ is a closed nonorientable $4$-manifold which admits a Lefschetz fibration over the $2$-sphere, equipped with a section of square $\pm 1$, then we construct a trisection diagram of $X$, which is determined by the vanishing cycles of the Lefschetz fibration. Finally, we include some simple observations about low-genus Lefschetz fibrations on closed nonorientable $4$-manifolds.