**Extended Abstract:**

- Shelah proved that Cohen forcing introduces a Souslin tree;
- Jensen proved that a
*c.c.c.*forcing may consistently add a Kurepa tree; - Todorcevic proved that a Knaster poset may already force the Kurepa hypothesis;
- Irrgang introduced a
*c.c.c.*notion of forcing based on a simplified ($\omega_1$,1)-morass that adds an $\omega_2$-Souslin tree.

Here, it is proved that adding a subset of $\omega_2$ may introduce a special Aronszajn tree of height $\aleph_{\omega_1+1}$ :

Starting with a model of two supercompact cardinals, we construct a model with no special $\aleph_{\omega_1+1}$-Aronszajn trees, in which there exists a notion of forcing $\mathbb P$ of cardinality $\omega_3$, which is $\sigma$-closed, $\omega_1$-distributive, $\omega_3$-Knaster, and such that in the generic extension by $\mathbb P$, there exists a special Aronszajn tree of height $\aleph_{\omega_1+1}$.

**Abstract:**

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**Citation information:**

A. Rinot, *A cofinality-preserving small forcing may introduce a special Aronszajn tree*, Arch. Math. Logic, 48(8): 817-823, 2009.