Contact de Rham cohomology and Hodge structures transversal to the Reeb foliations

Abstract: Let $\beta$ be a contact form on a compact smooth manifold $X$ and $v_\beta$ its Reeb vector field. The paper applies general results of different authors about Hodge structures that are transversal to a given foliation to the special case of $1$-dimensional foliation generated by the Reeb flow $v_\beta$. Theses applications are available for the Reeb flows on {\sf closed} manifolds $X$. In contrast, for the Reeb flows on manifolds with boundary, little is known about the Hodge structures transversal to the $v_\beta$-flow. We are trying to fill in this gap.\smallskip The de Rham differential complex $\Omega_{\mathsf b}^\ast(X, v_\beta)$ of, so called, {\sf basic} relative to $v_\beta$-flow differential forms is in the focus of this investigation. By definition, the basic forms vanish when being contracted with $v_\beta$, and so do their differentials. In particular, we investigate when the $2$-form $d\beta$ and its powers deliver nontrivial elements in the basic de Rham cohomology $H^\ast_{\mathsf{basic}\,d\mathcal{R}}(X, v_\beta)$ of the differential complex $\Omega_{\mathsf b}^\ast(X, v_\beta)$. Answers to these questions seem to contrast sharply the cases of a closed $X$ and a $X$ with boundary. %we prove that when a $v_\beta$-flow admits a Lyapunov function, then the basic de Rham cohomology $H^\ast_{\mathsf{basic}\,d\mathcal{R}}(X, v_\beta)$ of the complex $\Omega_{\mathsf b}^\ast(X, v_\beta)$ are topological invariants of $X$. On the other hand, building on work of Ra\'{z}ny \cite{Raz}, we show that on closed manifolds, equipped with a transversal to the Reeb flow Hodge structure that satisfies the {\sf Basic Hard Lefschetz property}, the basic de Rham cohomology $H^\ast_{\mathsf{basic}\,d\mathcal{R}}(X, v_\beta)$ are topological invariants of $X$.