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(All my rings are commutative and unital.)

Definition. Call a totally-ordered ring $R$ special iff for all non-zero $b \in R,$ every coset of $bR$ has a unique element in the interval $[0,|b|).$

Motivation. This means that for any non-trivial principal ideal $bR$ of $R$, we have a natural bijective correspondence between $R/bR$ and $[0,|b|)$. We can, for example, use this to find the cardinality of $R/bR$.

Examples. The totally-ordered ring $\mathbb{Z}$. Hence $\mathbb{Z}/a\mathbb{Z}$ has the same cardinality as $[0,|a|),$ for all non-zero $a$. Hence $|\mathbb{Z}/a\mathbb{Z}|=|a|,$ for all non-zero $a$.

In fact, this is the only example I can think of. Mini-question: what are some other examples of totally-ordered rings that are special?

Question. Is $\mathbb{Z}$ is the only totally ordered PID that is special?

goblin GONE
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  • Do you have an example of a special ring other than $\mathbb Z$? – Mostafa - Free Palestine Aug 13 '15 at 19:16
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    What do $|b|$ and $[u,v)$ mean in a general context? This seems like you are assuming an embedding into $\mathbb R$ but then I do not know any theory on ordered rings. – blue Aug 13 '15 at 22:30
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    @blue, in any totally ordered ring, we define $|b| = \mathrm{max}(b,-b).$ As for $[u,v)$... well, given elements $u,v \in P$ where $P$ is an arbitrary poset, what do you think this notation is intended to mean? – goblin GONE Aug 14 '15 at 05:35
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    Note that if you relax "PID" to "Bezout domain", then the condition "special totally ordered Bezout domain" can be expressed in the first-order language of totally ordered rings, so there are many such examples. – Eric Wofsey Aug 15 '15 at 15:40
  • @EricWofsey, there are? Some kind of crazy ultrapower thingo perhaps? – goblin GONE Aug 15 '15 at 15:45
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    One obvious observation, such a ring can't have any units other than $\pm 1$, as otherwise you could find two different intervals, one strictly containing the other, each of which somehow formed a system of coset representatives for $bR$... – Harry Altman Aug 15 '15 at 19:24
  • For a very simple-to-understand example of a special totally ordered ring larger than $\mathbb{Z}$ (though not a PID), you can take the ring of polynomials in $\mathbb{Q}[x]$ whose constant term is in $\mathbb{Z}$, ordered such that $x$ is infinitely large. This is the ring $R_\alpha$ from my answer in the case $\alpha=0$. – Eric Wofsey Aug 16 '15 at 06:54
  • @goblin: yes; this follows from the upward Lowenheim-Skolem theorem, which implies that if a first-order theory has an infinite model then it has models of every larger cardinality. Ultrapowers are one way to construct some of these. – Qiaochu Yuan Jan 30 '16 at 08:57

1 Answers1

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Here is a counterexample. Let $\hat{\mathbb{Z}}$ be the profinite completion of $\mathbb{Z}$. Say that an element $\alpha\in\hat{\mathbb{Z}}$ is Noetherian if for any nonzero polynomial $f(x)\in \mathbb{Z}[x]$, $f(\alpha)$ is divisible by only finitely many elements of $\mathbb{Z}$. For any $\alpha\in\hat{\mathbb{Z}}$, let $R_\alpha\subset\mathbb{Q}[x]$ be the ring of polynomials of the form $f(x)/n$, where $f(x)\in\mathbb{Z}[x]$, $n\in \mathbb{Z}$, and $n$ divides $f(\alpha)$. Order $R_\alpha$ by declaring that $x$ is infinitely large.

First, I claim that one can carry out a sort of Euclidean algorithm with elements of $R_\alpha$. Suppose we have positive elements $a,b\in R_\alpha$ with $b<a$. First, suppose $\deg b<\deg a$. By the division algorithm in $\mathbb{Q}[x]$, there is some $\tilde{c}\in\mathbb{Q}[x]$ such that $\tilde{r}=a-\tilde{c}b$ has degree smaller than $b$. There is some $q\in\mathbb{Q}$ such that $0\leq q<1$ and $c=\tilde{c}-q\in R_\alpha$ (this is because if $N\in\mathbb{Z}$ is such that $N\tilde{c}\in \mathbb{Z}[x]$, then $N\tilde{c}(\alpha)-n$ must be divisible by $N$ for some $n\in[0,N)$). Then $r=a-cb=\tilde{r}+qb$ satisfies $0\leq r<b$.

On the other hand, suppose $\deg b=\deg a$. Then we can just perform the Euclidean algorithm for $\mathbb{Z}$ paying attention only to the leading coefficients of $a$ and $b$, and in finitely many steps we will have reduced the degree of one of them.

By following the above algorithm, we can always reduce the degree of one of our two elements of $R_\alpha$ through finitely many steps. Thus the algorithm must always terminate, so we obtain a common divisor of any two elements $a,b\in R_\alpha$ which is in the ideal $(a,b)$. It follows that $R_\alpha$ is a Bezout domain.

Furthermore, $R_\alpha$ is special. Indeed, we proved the existence part of the definition of "special" in the course of describing the Euclidean algorithm for $R_\alpha$. For the uniqueness part, just note that there is no element of $R_\alpha$ between $0$ and $1$, so if $0\leq r<s<b$ then $s-r$ cannot be divisible by $b$.

Finally, suppose $\alpha$ is Noetherian. Then I claim that there are no infinite chains $a_1>a_2>\dots$ of positive elements of $R_\alpha$ such that $a_{n+1}\mid a_n$ for each $n$; it follows from this that $R_\alpha$ is actually a PID. Indeed, in any such chain, the degree of $a_n$ can only go down finitely many times, so the quotients $a_n/a_{n+1}$ must eventually all be in $\mathbb{Z}$. But since $\alpha$ is Noetherian, $a_1$ cannot be divisible by infinitely many integers, so this is impossible.

It remains to be shown that there exists a Noetherian element $\alpha\in\hat{\mathbb{Z}}$. To show this, enumerate the nonzero elements of $\mathbb{Z}[x]$ as $\{f_n\}$. Note that for any $n$, there exists a number $N_n$ such that if $p>N_n$ is prime, then there is some $a\in\mathbb{Z}/(p)$ such that $f_i(a)\neq0\pmod p$ for $i=1,\dots,n$. We can thus choose the residues of $\alpha$ mod $p$ such that whenever $p>N_n$, $f_i(\alpha)$ is not divisible by $p$ for all $i\leq n$. For each $n$, then, $f_n(\alpha)$ can only be divisible by finitely many distinct primes.

Now for $p$ fixed, suppose we have already chosen the residue of $\alpha$ mod $p^d$ for some $d$. For any $n$, we can find a residue $a\in\mathbb{Z}/(p^e)$ for some $e\gg d$ lifting our residue mod $p^d$ such that $f_n(a)$ is not divisible by $p^e$ (because if we choose $a$ to be small relative to $p^e$ the only way $f_n(a)$ can be divisible by $p^e$ is if $f_n(a)=0$, and $f_n$ has only finitely many integer roots). So we can one by one choose residues of $\alpha$ mod $p^d$ for higher and higher $d$ such that each $f_n(\alpha)$ ends up divisible by $p$ only finitely many times.


As a final note, this construction is in some sense the only way to get a counterexample. Indeed, if $R$ is any special totally ordered ring and $x\in R\setminus \mathbb{Z}$ is positive, $x$ must be infinitely large and its reductions mod $n$ for each $n\in\mathbb{Z}$ determine an element $\alpha\in\hat{\mathbb{Z}}$. It is then easy to see that $R$ must contain the subring $R_\alpha\subset \mathbb{Q}[x]$. If in addition $R$ is Noetherian, then $\alpha$ must be Noetherian. So every special totally ordered (Noetherian) ring besides $\mathbb{Z}$ is a union of subrings of the form $R_\alpha$ (for $\alpha$ Noetherian).

Eric Wofsey
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