UNIVERSAL COMPUTABLY ENUMERABLE EQUIVALENCE RELATIONS URI ANDREWS, STEFFEN LEMPP, JOSEPH S. MILLER, KENG MENG NG, LUCA SAN MAURO, AND ANDREA SORBI Abstract. We study computably enumerable equivalence relations (ceers), under the reducibility R ≤ S if there exists a computable function f such that x R y if and only if f(x) S f(y), for every x; y. We show that the degrees of ceers under the equivalence relation generated by ≤ form a bounded poset that is neither a lower semilattice, nor an upper semilattice, and its first order theory is undecidable. We then study the universal ceers. We show that 1) the uniformly effectively inseparable ceers are universal, but there are effectively inseparable ceers that are not universal; 2) a ceer R is universal if and only if R0 ≤ R, where R0 denotes the halting jump operator introduced by Gao and Gerdes (answering an open question of Gao and Gerdes); and 3) both the index set of the universal ceers and the index set of the uniformly effectively 0 inseparable ceers are Σ3-complete (the former answering an open question of Gao and Gerdes). 1. Introduction Given equivalence relations R and S over the set ! of the natural numbers, define R reducible to S (notation: R ≤ S) if there exists a computable function f such that x R y , f(x) S f(y), for all x; y. This reducibility can be viewed as a natural computable version of Borel reducibility (in which the reduction function is Borel), widely studied in descriptive set theory to measure the relative complexity of Borel equivalence relations on Polish spaces, see for instance the textbooks [1, 19]. The reducibility ≤ was introduced by Ershov (see Ershov's monograph [13], translated into German in [10, 11, 12]), while dealing with the monomorphisms of a certain category. Following Ershov, one can introduce the category of equiv- alence relations on !, in which a morphism from R to S is a function µ : !=R −! !=S between the corresponding quotient sets, such that there is a computable function f with µ([x]=R) = [f(x)]=S, or in other words, x R y ) f(x) S f(y). 2010 Mathematics Subject Classification. 03D25. Key words and phrases. Computably enumerable equivalence relations, effectively inseparable sets. Lempp's research was partially supported by an AMS-Simons Foundation Collaboration Grant 209087. Miller's research was partially supported by NSF grant DMS-1001847. Sorbi's research was partially carried out while he was visiting the Department of Mathematics of the University of Wisconsin-Madison in March 2011. The second and last author would like to thank the Isaac Newton Institute for Mathematical Sciences of Cambridge, England, for its hospitality during the final phase this paper was completed. 1 2 ANDREWS, LEMPP, MILLER, NG, SAN MAURO, AND SORBI (In fact, Ershov's primary interest is in the category of numberings, of funda- mental interest in the foundations of computability theory, computable algebra, and computable model theory. One can view the category of equivalence rela- tions as a full subcategory of the category of numberings, such that every object in the category of numberings is isomorphic to an object in the subcategory of equivalence relations.) It is then easy to see that R ≤ S if and only if, in the terminology of category theory, there is a monomorphism from R to S, i.e., R is a subobject of S. In recent years, there has been a sudden burst of interest in the reducibility ≤, either to compare it and contrast it with Borel reducibility (see for instance [8, 18, 15]), or to study the complexity of various equivalence relations, such as the isomorphism relation, on classes of computable structures (see for instance [14, 16, 17]). We study computably enumerable equivalence relations (for short, ceers) under ≤. Ceers appear frequently in mathematics (for instance as equality of words in finitely presented semigroups or groups), in computable model theory (see e.g., 0 [6], where ceers are called Σ1 equivalence structures), and in the theory of num- berings, where the ceers are exactly the equivalence relations corresponding to the so called positive numberings (in fact, in the Russian literature, ceers are often called positive equivalence relations, as in [9]). Applications of the reducibility ≤ to ceers have also been motivated by proof theoretic interests in the relation of provable equivalence in formal systems (see for instance [4, 3, 25, 33]), and in universal ceers, i.e., ceers which are complete under ≤ with respect to the class of all ceers. An important result along these lines was the discovery that the class of universal ceers contain distinct computable isomorphism types, one of which is constituted by the precomplete ceers, proven by Lachlan [22]. We prove some useful facts about universal ceers and the poset of degrees of ceers under the reducibility ≤. In particular, in Section 2 we show that the poset of degrees of ceers under ≤ is neither an upper semilattice nor a lower semilattice (Corollary 2.5), and has undecidable first order theory (Theorem 2.4). We then turn to the study of universal ceers, and in Section 3 we show (Corollary 3.16) that every u.e.i. ceer is universal, where a u.e.i. ceer is a nontrivial ceer (i.e., with more than one equivalence class) providing a partition of ! into sets that are uniformly effectively inseparable; this extends all known universality results for ceers, and is a natural extension to ceers of classical results stating universality of creative sets, and pairs of effectively inseparable sets. We prove that uniformity is essential to conclude universality, since in Theorem 3.19 we give an example of a ceer that is not universal, but yields a partition into effectively inseparable sets; our proof of universality of u.e.i. ceers succeeds by showing the identity of the class of u.e.i. ceers with certain classes of ceers, obtained by refining classes already known in the literature. In Section 4 we study the halting jump operation on ceers introduced by Gao and Gerdes [20]: Answering an open question raised in [20], we show (Theorem 4.3) that a ceer is universal if and only if it is bi- reducible with its halting jump. In the final Section 5 we study some index sets of classes of universal ceers, and answering an open question raised again by Gao and Gerdes [20], we show (Theorem 5.1) that the index set of universal ceers is 0 Σ3-complete. UNIVERSAL CEERS 3 1.1. Background. Our terminology and notations about computability theory are standard, and can be found for instance in the textbooks [29, 32]. A clear and thorough introduction to ceers is provided by [20], which is currently the most complete attempt to give a systematic study of ceers under ≤. For later reference, we show how ceers can be computably numbered, and approximated. We say that a numbering ν of the ceers (i.e., a function ν from ! onto the ceers) is computable if fhe; x; yi : x ν(e) yg is a c.e. set. One natural way to number all ceers is via the following construction. For every set of numbers X, let X∗ denote the equivalence relation on ! generated by X, where of course we view X as a subset of !2, via the Cantor pairing function. It is easy to see that there exists a ∗ computable function γ such that, Wγ(e) = We , for every e, and if We is already an equivalence relation on !, then Wγ(e) = We. Then the numbering of all ceers, ν(e) = Wγ(e), is computable. Moreover, it is easy to see that ν is universal, or principal, i.e., for every computable numbering ρ of all ceers, there exists a computable function f such that ρ = ν ◦ f. (For different universal computable numberings of all ceers, see e.g., [9, 20].) Throughout the rest of the paper, we denote Re = ν(e). We say that a sequence fRs : s 2 !g of equivalence relations on ! is a com- putable approximation to a ceer R, if (1) the set fhx; y; si : x Rs yg is computable; (2) R0 = Id; (3) for all s, Rs ⊆ Rs+1; the equivalence classes of Rs are finite; there exists at most one pair [x]Rs ; [y]Rs of equivalence classes, such that [x]Rs \[y]Rs = ;, but [x]Rs+1 = [y]Rs+1 (we say in this case that the equivalence relation R-collapses x and y at stage s + 1); S t (4) R = t R . s Lemma 1.1. There exists a sequence fRe : e; s 2 !g of equivalence relations s s such that fhe; x; y; si : x Re yg is computable, and the sequence fRe : s 2 !g is a computable approximation to Re. Therefore an equivalence relation R is a ceer if and only if R can be computably approximated. Moreover if R is a ceer and R n fhx; xi : x 2 !g is infinite, then one can find an approximating sequence fRs : s 2 !g to R satisfying that for every s, the relation Rs+1 is obtained from Rs by the R-collapse of exactly one pair of equivalence classes of Rs. Proof. Straightforward. 2. Universal ceers and the poset of degrees of ceers The following definition plays a crucial role in this paper: Definition 2.1. A ceer R is universal if S ≤ R, for every ceer S. Clearly, universal ceers do exist: For instance, the ceer R where hi; xi R hj; yi if i = j and x Ri y, is clearly universal.