Pi-one formula in set theory
= Pi-one formula in set theory
{title2=$\Pi_1$}
= Pi-one formulas in set theory
{synonym}
A syntactically $\Pi_1$ <first-order formula> has an unbounded universal quantifier block followed by a <bounded formula in set theory>. Universal quantification makes it downward absolute between <transitive models>. A <Pi-one formula modulo ZF> may have another syntax but has such an equivalent provable in <ZF>.