An almost Kurepa Suslin tree with strongly non-saturated square

Abstract

For uncountable downwards closed subtrees $U$ and $W$ of an $\omega_1$-tree $T$, we say that $U$ and $W$ are strongly almost disjoint if their intersection is a finite union of countable chains. The tree $T$ is strongly non-saturated if there exists a strongly almost disjoint family of $\omega_2$-many uncountable downwards closed subtrees of $T$. In this article we construct a Knaster forcing which adds a Suslin tree together with a family of $\omega_2$-many strongly almost disjoint automorphisms of it (and thus the square of the Suslin tree is strongly non-saturated). To achieve this goal, we introduce a new idea called $\rho$-separation, which is an adaptation to the finite context of the notion of separation which was recently introduced by Stejskalov\'{a} and the first author for the purpose of adding automorphisms of a tree with a forcing with countable conditions.