On the minimal sum of Betti numbers of an almost complex manifold

Abstract

We show that the only rational homology spheres which can admit almost complex structures occur in dimensions two and six. Moreover, we provide infinitely many examples of six-dimensional rational homology spheres which admit almost complex structures, and infinitely many which do not. We then show that if a closed almost complex manifold has sum of Betti numbers three, then its dimension must be a power of two.

9 pages, 1 figure, to appear in Differential Geometry and its Applications