Abstract

Let $\mathscr{M}$ be a structure for a language $\mathscr{L}$ on a set $M$ of urelements. $\mathrm{HYP}(\mathscr{M})$ is the least admissible set above $\mathscr{M}$. In $\S 1$ we show that $pp(\mathrm{HYP}(\mathscr{M})) \lbrack = \text{the collection of pure sets in} \mathrm{HYP}(\mathscr{M}\rbrack$ is determined in a simple way by the ordinal $\alpha = \circ(\mathrm{HYP}(\mathscr{M}))$ and the $\mathscr{L}_{\propto\omega}$ theory of $\mathscr{M}$ up to quantifier rank $\alpha$. In $\S 2$ we consider the question of which pure countable admissible sets are of the form $pp(\mathrm{HYP}(\mathscr{M}))$ for some $\mathscr{M}$ and show that all sets $L_\alpha (\alpha$ admissible) are of this form. Other positive and negative results on this question are obtained