Finite groups acting symplectically on $T^2\times S^2$

Abstract:

For any symplectic form $\omega$ on $T^2\times S^2$ we construct infinitely many nonisomorphic finite groups which admit effective smooth actions on $T^2\times S^2$ that are trivial in cohomology but which do not admit any effective symplectic action on $(T^2\times S^2,\omega )$. We also prove that for any $\omega$ there is another symplectic form $\omega ’$ on $T^2\times S^2$ and a finite group acting symplectically and effectively on $(T^2\times S^2,\omega ’)$ which does not admit any effective symplectic action on $(T^2\times S^2,\omega )$.

A basic ingredient in our arguments is the study of the Jordan property of the symplectomorphism groups of $T^2\times S^2$. A group $G$ is Jordan if there exists a constant $C$ such that any finite subgroup $\Gamma$ of $G$ contains an abelian subgroup whose index in $\Gamma$ is at most $C$. Csikós, Pyber and Szabó proved recently that the diffeomorphism group of $T^2\times S^2$ is not Jordan. We prove that, in contrast, for any symplectic form $\omega$ on $T^2\times S^2$ the group of symplectomorphisms $\mathrm {Symp}(T^2\times S^2,\omega )$ is Jordan. We also give upper and lower bounds for the optimal value of the constant $C$ in Jordan’s property for $\mathrm {Symp}(T^2\times S^2,\omega )$ depending on the cohomology class represented by $\omega$. Our bounds are sharp for a large class of symplectic forms on $T^2\times S^2$.