QUASI-EUCLIDEAN SUBRINGS OF Q[x] PETR GLIVICKY´ AND JAN SAROCHˇ Abstract. Using a nonstandard model of Peano arithmetic, we show that there are quasi-Euclidean subrings of Q[x] which are not k-stage Euclidean for any norm and positive integer k. These subrings can be either PID or non-UFD, depending on the choice of parameters in our construction. In both cases, there are 2ω such domains up to ring isomorphism. Although Euclidean and principal ideal domains have been intensively studied for almost a century, examples of non-Euclidean PIDs are still rather scattered throughout the literature, and thought of as more or less singular, non-frequent objects. The oldest of these examples are arguably the rings of integers of Q(√d) for d = 19, 43, 67, 163. However, these are the only cases for negative d’s, and the results− − from− [12]− and [7] indicate that it is almost surely the case of positive values of d, too. Another type of examples was given by Samuel in his famous paper [11]. Leut- becher (in [8]) capitalized on his approach several years later, and proved that there are non-Euclidean PIDs which are even quasi-Euclidean (this was not the case of the four rings of integers mentioned above, as Cohn observed in [4]). Throughout this paper, by a quasi-Euclidean domain, we mean a commutative domain R for which there is a function φ : R2 ω such that, for all (a,b) R2 with b = 0, there exists q R with φ(b,a →bq) < φ(a,b). The definition∈ is similar to6 the one of classical∈ Euclidean norm,− with the important difference that by the norm function here, we do not measure elements of the ring but pairs of those. Also, unlike in the case of Euclidean domains, ω can be equivalently replaced by some/any infinite ordinal in the definition; see Preliminaries section (in particular Proposition 1.1) for this and further equivalent definitions of quasi- Euclidean domain, and related concepts. There are a few more published results on non-Euclidean PIDs. Unfortunately, they do not usually present a coherent class of these domains, or some sort of characterization of rings which are non-Euclidean PIDs in some distinguished class of domains. Nice attempts in this direction can be found in [1] and [6]. arXiv:1410.6746v1 [math.RA] 24 Oct 2014 In this paper, we present a parametric construction which is in some sense a generalization of the approach used in [6]. While studying certain models of Peano arithmetic, we noticed that there are many discretely ordered non-Euclidean (even non-k-stage Euclidean in the sense of Cooke [5]) subrings of Q[x] which are quasi- Euclidean. In fact, for each τ Jp, where Jp denotes the ring of p-adic ∈ p∈P integers, we define one such subring. Moreover, we observe that the set J Q p∈P p splits into two parts of full cardinalities, depending on whether the resulting ring Q Date: July 8, 2018. 2010 Mathematics Subject Classification. 13F07 (primary), 13F10, 13F20, 03H15 (secondary). Key words and phrases. k-stage division chain, quasi-Euclidean domain, PID. First author supported by the grant GAUK 4372/2011. Second author supported by the grant ECCˇ 301-29/248001. 1 2 PETR GLIVICKY´ AND JAN SAROCHˇ is PID or non-UFD. Since each quasi-Euclidean ring is B´ezout (Proposition 1.1), there are no inbetween cases, i.e. non-PID and UFD at the same time. Acknowledgements. The authors would like to thank Josef Mlˇcek and Jan Trlifaj for reading parts of this text and giving several valuable comments. 1. Preliminaries Throughout this paper, all rings are (commutative integral) domains. Further, we denote by P the set of all primes in N. For each p P, Jp stands for the ∈ ring of p-adic integers, while Zp denotes the field Z/pZ. Since Jp = lim Zpk , we ∞ ∼ shall view Jp as a subring of k=1 Zpk , and denote, for a positive integer←−k, by πk the canonical projection from Jp to Zpk . It will not cause any confusion that the Q notation πk does not reflect the prime p. Moreover, for technical reasons, we put π0 : Jp 0 ; again, regardless of the prime p. If we→{ deal} with elements from the ring Q[x], we define deg 0 = 1, and we denote by lc(q) the leading coefficient of a polynomial q. − 1.1. Quasi-Euclidean and k-stage Euclidean domains. Various generaliza- tions of the concept of a Euclidean domain were proposed and studied in the past. The one we find very natural, is the concept of quasi-Euclidean (used in [8] and [3]) or the equivalent notion of ω-stage Euclidean domain (used by Cooke in [5]). Given a ring R and a partial order on R2, we say that is quasi-Euclidean if it has the descending chain condition≤ (dcc), and for any pair≤ (a,b) R2 with b = 0, there exists an element q in R such that (b,a bq) < (a,b).∈ We call R quasi-Euclidean6 provided there exists a quasi-Euclidean− partial order on R2. Let (a,b) R2 and k be a non-negative integer. A k-stage division chain starting from the pair∈ (a,b) is a sequence of equations in R a = q1b + r1 b = q2r1 + r2 r1 = q3r2 + r3 . rk−2 = qkrk−1 + rk. Such a division chain is called terminating if the last remainder rk is 0 (rk−1 is then easily seen to be the GCD of a and b). Notice that a k-stage division chain is determined by its starting pair and the sequence of quotients q1,...,qk. For the sake of compactness, in what follows, we shall denote this chain also by a q ... q 1 k . b r ... rk 1 Given such a division chain, we define its 0-th remainder r0 as b. In the following proposition, On denotes the class of all ordinal numbers. Proposition 1.1. ([3], [5], [8]) For a commutative domain R, the following condi- tions are equivalent: (1) There exists a function φ : R2 On (with Rng(φ) ω) such that, for all (a,b) R2 with b =0, there exists→ q R such that φ⊆(b,a bq) < φ(a,b). (2) R is quasi-Euclidean.∈ 6 ∈ − (3) R is a B´ezout domain, and the group GL2(R) of regular 2 2 matrices over R is generated by matrices of elementary transformations.× (4) Every pair (a,b) R2 with b =0 has a terminating k-stage division chain for some positive∈ integer k. 6 QUASI-EUCLIDEAN SUBRINGS OF Q[x] 3 Proof. (1) = (2) is trivial, we just put (a,b) < (a′,b′) if φ(a,b) < φ(a′,b′). (2) = (4)⇒ follows directly by the dcc. The equivalence⇒ of (3) and (4) was proved already in [9, 14.3]. (4) = (1): We put φ(a, 0) = 0 for all a R. If b = 0, we define φ(a,b) as the minimal⇒k ω for which the pair (a,b) has∈ a terminating6 k-stage division chain. (So we even∈ manage to find φ with the range in ω.) Notice that no notion of a norm is involved in the definition of a quasi-Euclidean domain. However, given a norm N on R (i.e. a function N : R N with N(a)=0 iff a = 0), we can measure how far N is from being Euclidean:→ as in [5], for 0 < k ω, we say that R is a k-stage Euclidean domain with respect to N provided that, for≤ every (a,b) R2 with b = 0, there exists a positive integer l k such that ∈ 6 ≤ for some l-stage division chain starting from (a,b) it is N(rl) < N(b). As usual, we say that R is k-stage Euclidean if there exists such a norm N on R. So, in our notation, 1-stage Euclidean means Euclidean (in the classic sense). On the other hand, by Proposition 1.1, R is ω-stage Euclidean (with respect to some/any norm) if and only if it is quasi-Euclidean. Finally, observe that a quasi-Euclidean domain, being B´ezout, is UFD if and only if it is PID. An example of non-UFD 2-stage Euclidean domain was given already by Cooke in [5], at the end of 1. It is at this place, where he admits that he does not know of any example of quasi-Euclidean§ domain which is not 2-stage Euclidean. Interestingly, all examples, we are going to construct, have got this property. 1.2. Peano arithmetic and weak saturation. Although our construction will be purely algebraic, we are going to give also a description derived from a nonstan- dard model of Peano arithmetic (PA). There are several reasons to do this: the description is very natural, only basic logical tools are needed, and it sheds more light at the entire situation. Our models of PA are thought of as models in the language L = (0, 1, +, , ). The fact that it is an extension of the language of rings will make it more convenient· ≤ for us to work with. In particular, we can immediately say that any model of PA is a (discretely ordered) commutative semiring with 0 and 1. We will say that = PA is weakly saturated if every 1-type in without M | M parameters is realized in , i.e. given any set Y = ϕi(x) i I of L-formulas M { | ∈ } with one free variable x, there is m M such that = ϕi[m] for all i I, ∈ M | ∈ provided that, for each finite subset S of I, one has = ( x) i∈S ϕi(x).