A countable equivalence relation of second projective class not generated by projective functions

Published 4 May 2026 in math.LO | (2605.03126v1)

Abstract: To answer a question by Rettich and Serafin, we define a model of set theory in which there exists a countable $Π^1_2$ equivalence relation on a subset of the real line, which is not generated by a countable family of projective (or even ROD) functions.

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