Delta-one formula in set theory
= Delta-one formula in set theory
{title2=$\Delta_1^{\mathrm{ZF}}$}
A formula is $\Delta_1^{\mathrm{ZF}}$ when ZF proves it equivalent both to a $\Sigma_1$ formula and to a $\Pi_1$ formula. Such formulas are absolute between suitable transitive models of finite fragments of ZF.