Full mad families of vector spaces and two local Ramsey theories

Abstract

Let $E$ be a vector space over a countable field of dimension ${\aleph }_0$. Two infinite-dimensional subspaces $V,W\subseteq E$ are almost disjoint if $V\cap W$ is finite-dimensional. This paper provides some improvements on results about the definability of maximal almost disjoint families (mad families) of subspaces in [18]. We construct a full mad family of block subspaces in ZFC, answering a problem by Smythe in the positive. A variant of this construction shows that there exists a completely separable mad family of block subspaces in ZFC. We also discuss the abstract Mathias forcing introduced by Di Prisco-Mijares-Nieto in [12], and apply it to show that in the Solovay's model obtained by the collapse of a Mahlo cardinal, there are no full mad families of block subspaces over ${\mathbb{F}}_2$.

50 pages. Final accepted version in Fundamenta Mathematicae