Relativization of Gurevich's Conjectures

Abstract: Gurevich (1988) conjectured that there is no logic for $\textsf{P}$ or for $\textsf{NP}\cap \textsf{coNP}$. For the latter complexity class, he also showed that the existence of a logic would imply that $\textsf{NP} \cap \textsf{coNP}$ has a complete problem under polynomial time reductions. We show that there is an oracle with respect to which $\textsf P$ does have a logic and $\textsf P \ne\textsf{NP}$. We also show that a logic for $\textsf{NP} \cap \textsf{coNP}$ follows from the existence of a complete problem and a further assumption about canonical labelling. For intersection classes $\Sigma^p_n \cap \Pi^p_n$ higher in the polynomial hierarchy, the existence of a logic is equivalent to the existence of complete problems.