Obstructions for exact submanifolds with symplectic applications

Abstract

Suppose $X^N$ is a closed oriented manifold, $\alpha \in H^{\ast }(X;\mathbb{R})$ is a cohomology class, and $Z\in H_{N-k}(X)$ is an integral homology class. We ask the following question: is there an oriented embedded submanifold $Y^{N-k}\subset X$ with homology class $Z$ such that $\alpha {|}_Y=0\in H^{\ast }(Y;\mathbb{R})$? In this article, we provide a family of computable obstructions to the existence of such 'exact' submanifolds in a given homology class which arise from studying formal deformations of the de Rham complex. In the final section, we apply these obstructions to prove that the following symplectic manifolds admit no non-separating exact (a fortiori contact-type) hypersurfaces: Kähler manifolds, symplectically uniruled manifolds, and the Kodaira--Thurston manifold.