A $wtt$-introimmune set in \texorpdfstring{$Π^0_1$}{Pi01} and introimmunity for several reducibilities

Abstract

We prove that there exists a weak truth-table introimmune set in the class $Π^0_1$, settling the question left open in previous work of whether the known $Δ^0_2$ existence result can be improved to $Π^0_1$. Since $Σ^0_1$ sets cannot be immune, this is best possible for weak truth-table introimmunity. We also study introimmunity for Jockusch's bounded-search reducibility $\le_{bs}$ and Andersen's Dartmouth reducibility $\le_D$, proving the existence of $Δ^0_2$ sets that are $bs$-introimmune and $D$-introimmune; hence there also exists a $Δ^0_2$ $D^+$-introimmune set. We next consider the classical reducibility $\le_Q$, which is not contained in $\le_T$ on all subsets of $ω$. We show that no infinite $Π^0_1$ set is $Q$-introimmune, while a $Δ^0_2$ $Q$-introimmune set does exist. Thus the existence of $Δ^0_2$ $Q$-introimmune sets is best possible within the arithmetical hierarchy. Finally, for enumeration reducibility $\le_e$, we show that no infinite $Π^1_1$ set is $e$-introimmune, although $e$-introimmune sets do exist in the unrestricted sense. The proofs combine finite-injury priority arguments with dynamic spacing methods for $\le_{wtt}$, $\le_{bs}$, and $\le_D$, a bit-by-bit finite-extension construction for $\le_Q$, and an application of Soare's abstract existence theorem in the enumeration case.