Mathematical Research Letters

Volume 29 (2022)

Number 1

On contact invariants of non-simply connected Gorenstein toric contact manifolds

Pages: 1 – 42

DOI: https://dx.doi.org/10.4310/MRL.2022.v29.n1.a1

Authors

Miguel Abreu (Center for Mathematical Analysis, Geometry and Dynamical Systems, Instituto Superior Técnico, Universidade de Lisboa, Portugal)

Leonardo Macarini (Center for Mathematical Analysis, Geometry and Dynamical Systems, Instituto Superior Técnico, Universidade de Lisboa, Portugal)

Miguel Moreira (Department of Mathematics, ETH Zürich, Switzerland)

Abstract

The first two authors showed in [1] how the Conley–Zehnder index of any contractible periodic Reeb orbit of a non-degenerate toric contact form on a good toric contact manifold with zero first Chern class, i.e. a Gorenstein toric contact manifold, can be explicitly computed using moment map data. In this paper we show that the same explicit method can be used to compute Conley–Zehnder indices of non-contractible periodic Reeb orbits. Under appropriate conditions, the (finite) number of such orbits in a given free homotopy class and with a given index is a contact invariant of the underlying contact manifold. We compute these invariants for two sets of examples that satisfy these conditions: $5$‑dimensional contact manifolds that arise as unit cosphere bundles of $3$‑dimensional lens spaces, and $2n + 1$‑dimensional Gorenstein contact lens spaces. As applications, we will see that these invariants can be used to show that diffeomorphic lens spaces might not be contactomorphic and that there are homotopy classes of diffeomorphisms of some lens spaces that do not contain any contactomorphism. Following a suggestion by one referee, we will also see that this type of applications can be proved alternatively by looking at the total Chern class of these canonical contact structures on lens spaces.

Received 19 March 2019

Received revised 10 January 2022

Accepted 19 January 2022

Published 6 September 2022