arXiv subjects

Rei Henigman

Publications and source records attributed to Rei Henigman.

3 recordsLinked to original sources

arXiv2026

Symplectic torus actions with non-contractible orbits

We prove that a symplectic $T^{n-1}$ action on a closed connected $2n$-dimensional symplectic manifold is Hamiltonian if and only if its orbits are contractible. This generalizes a result of Lalonde--McDuff--Polterovich on four-manifolds and theorems of McDuff and Kim on existence of fixed points. When the orbits are isotropic, we prove a stronger variant of this result, which implies non-extendability of certain symplectic circle actions on 6-manifolds to symplectic $T^2$ actions. Moreover, we prove that a symplectic $T^{n-1}$ action with isotropic orbits always splits into a maximal Hamiltonian action and a locally-free action. We end by posing several open questions on the topology of symplectic torus actions.

math.SG↗

arXiv2024

Maximal symplectic torus actions

There are several different notions of maximal torus actions on smooth manifolds, in various contexts: symplectic, Riemannian, complex. In the symplectic context, for the so-called isotropy-maximal actions, as well as for the weaker notion of almost isotropy-maximal actions, we give classifications up to equivariant symplectomorphism. These classification results give symplectic analogues of recent classifications in the complex and Riemannian contexts. Moreover, we deduce that every almost isotropy-maximal symplectic torus action is equivariantly diffeomorphic to a product of a symplectic toric manifold and a torus, answering a question of Ishida. The classification theorems are consequences of Duistermaat and Pelayo's classification of symplectic torus actions with coisotropic orbits.

math.SG↗

arXiv2024

Classification of symplectic non-Hamiltonian circle actions on 4-manifolds

We classify symplectic non-Hamiltonian circle actions on compact connected symplectic 4-manifolds, up to equivariant symplectomorphisms. Namely, we define a set of invariants, show that the set is complete, and determine which values are attainable by constructing a space for each valid choice. We work under the assumption that the group of periods of the one-form $\iota_X \omega$ is discrete, which allows us to define a circle-valued Hamiltonian for the action, and apply tools from Karshon-Tolman's work on the classification of complexity one spaces. This assumption is always satisfied if the symplectic form is rational, or if the quotient space has first Betti number one.

math.SG↗