# Algebra Seminar talk

2017-10-13 (12:00!)

Fabiana **Castiblanco** (Universität Münster)*The ordinal $u_2$ and a $\bf{\Delta}^1_3$-thin equivalence relation*

Abstract:

Under the existence of sharps for reals the second uniform
indiscernible can be defined as $$u_2=\sup\{\omega_1^{+L[x]}:
x\in{}^\omega\omega\}$$ Suppose that $\mathbb{P}$ is Sacks, Mathias,
Silver, Miller or Laver forcing. In this talk we will see that if
every real has a sharp then $u_2^V=u_2^{V^\mathbb{P}}$ and, in fact,
$\mathbb{P}$ does not add any new equivalence class to the
$\bf{\Delta}^1_3$-thin equivalence relation defined by $xEy\iff
\omega_1^{+L[x]}=\omega_1^{+L[y]}$.