Infinitely many non-isotopic real symplectic forms on $S^2 \times S^2$

TL;DR

This paper demonstrates the existence of infinitely many non-isotopic real symplectic forms on the product of two spheres, revealing complex topological structures and introducing a novel diffeomorphism of the grassmannian with unique properties.

Contribution

It proves the disconnectedness of the space of monotone anti-invariant symplectic forms on a specific four-manifold and constructs a new diffeomorphism of the grassmannian with special homological properties.

Findings

The space of such symplectic forms is disconnected.

Existence of infinitely many non-isotopic symplectic forms.

Construction of a grassmannian diffeomorphism with identity on homology and homotopy groups.

Abstract

Let $(S^2,\omega)$ be a symplectic sphere, and let $\tau \colon S^2 \to S^2$ be an anti-symplectic involution of $(S^2,\omega)$. We consider the product $(S^2,\omega) \times (S^2,\omega)$ endowed with the anti-symplectic involution $\tau \times \tau$, and study the space of monotone anti-invariant symplectic forms on this four-manifold. We show that this space is disconnected. In addition, during the course of the proof, we produce a diffeomorphism of the grassmannian (2,4) which induces the identity map on all homology and homotopy groups, but which is not homotopic to the identity.