On Undecidability of Unary Nonnested PFP Operators for One Successor Function Theory

We investigate the decidability of first-order logic extensions. For example, it is established in Zolotov’s works that a logic with a unary transitive closure operator for the one successor theory is decidable. We show that in a similar case, a logic with a unary partial fixed point operator is undecidable. For this purpose, we reduce the halting problem for the counter machine to the problem of truth of the underlying formula. This reduction uses only one unary nonnested partial fixed operator that is applied to a universal or existential formula.