A K-contact simply connected 5-manifold with no semi-regular Sasakian structure

Abstract

We construct the first example of a 5-dimensional simply connected compact manifold that admits a K-contact structure but does not admit a semi-regular Sasakian structure. For this, we need two ingredients: (a) to construct a suitable simply connected symplectic 4-manifold with disjoint symplectic surfaces spanning the homology, all of them but one of genus 1 and the other of genus g>1, (b) to prove a bound on the second Betti number $b_2$ of an algebraic surface with $b_1=0$ and having disjoint complex curves spanning the homology when all of them but one are of genus 1 and the other of genus g>1.

32 pages, no figures; v2. added a proof that the symplectic 4-manifold in [MRT] cannot be complex; v3. added computation of second Stiefel-Whitney class; v4. small changes. Title changed