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Please read this first before answering. This question is only concerned with a proof of the dimension formula using the Coquand-Lombardi characterization below. If you post something that doesn't mention the characterization, then it's not an answer and is offtopic.


Background. If $R$ is a commutative ring, it is easy to prove $\dim(R[T]) \geq \dim(R)+1$, where $\dim$ denotes the Krull dimension. If $R$ is Noetherian, we have equality. Every proof I'm aware of uses quite a bit of commutative algebra and non-trivial theorems such as Krull's intersection theorem.

T. Coquand and H. Lombardi have found a surprisingly elementary characterization of the Krull dimension that does not use prime ideals at all.

T. Coquand, H. Lombardi, A Short Proof for the Krull Dimension of a Polynomial Ring, The American Mathematical Monthly, Vol. 112, No. 9 (Nov., 2005), pp. 826-829 (4 pages)

You can read the article here.

For $x \in R$ let $R_{\{x\}}$ be the localization of $R$ at the multiplicative subset $x^{\mathbb{N}} (1+xR) \subseteq R$. Then we have

$$\qquad \dim(R) = \sup_{x \in R} \left(\dim(R_{\{x\}})+1\right)\!. \label{1}\tag{$\ast$}$$

It follows that for $k \in \mathbb{N}$ we have $\dim(R) \leq k$ if and only if for all $x_0,\dotsc,x_k \in R$ there are $a_0,\dotsc,a_k \in R$ and $m_0,\ldots,m_k \in \mathbb{N}$ such that $$x_0^{m_0} (\cdots ( x_k^{m_k} (1+a_k x_k)+\cdots)+a_0 x_0)=0.$$ You can use this to define the Krull dimension.

A consequence of this is a new short proof of $\dim(K[x_1,\dotsc,x_n])=n$, where $K$ is a field. Using Noether normalization and the fact that integral extensions don't change the dimension, it follows that $\dim(R\otimes_K S)=\dim(R)+\dim(S)$ if $R,S$ are finitely generated commutative $K$-algebras. In particular $\dim(R[T])=\dim(R)+1$. This could be useful for introductory courses on algebraic geometry which don't want to waste too much time with dimension theory.

Question. Can we use the characterization \eqref{1} of the Krull dimension by Coquand-Lombardi above to prove $\dim(R[T])=\dim(R)+1$ for Noetherian commutative rings $R$?

Such a proof should not use the prime ideal characterization/definition of the Krull dimension. Notice that the claim is equivalent to $\dim(R[T]_{\{f\}}) \leq \dim(R)$ for all $f \in R[T]$.

Maybe this question is a bit naïve. I suspect that this can only work if we find a first-order property of rings which is satisfied by Noetherian rings and prove the formula for these rings. Notice that in contrast to that the Gelfand-Kirillov dimension satisfies $\mathrm{GK}\dim(R[T])=\mathrm{GK}\dim(R)+1$ for every $K$-algebra $R$.

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    Note: I've asked this on stackexchange http://math.stackexchange.com/questions/358423 and was encouraged to ask this here. – Martin Brandenburg Jun 21 '14 at 08:11
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    Another note: The characterization by T. Coquand and H. Lombardi is correct :-). – Martin Brandenburg Jun 24 '14 at 18:25
  • Yes, the statement is correct without any assumptions (I was mistaken). But still, it does not follow from what you can find in this article, which was the point. – Alex Gavrilov Jun 25 '14 at 12:13
  • I mean, the whole purpose was not to waste too much time with dimension theory. But this way you have to prove Corollary first, and it takes a little more then meets the eye. – Alex Gavrilov Jun 25 '14 at 12:28
  • @Alex: When you claim that the paper has a huge gap, you should say exactly(!) what is wrong. – Martin Brandenburg Jun 25 '14 at 13:28
  • @MartinBrandenburg: The alleged counterexample of Alex was puzzling. What was wrong finally? – ACL Jun 25 '14 at 15:57
  • @ACL: $\dim(C(S^1))>0$ since there are functions $f$ with $f^2 \nmid f$. (Unfortunately Alex deleted his example. My comments contained more elaborate explanations as well as references.) – Martin Brandenburg Jun 25 '14 at 20:55
  • @ACL: The example was interesting, but wrong: this ring has a positive Krull dimension. The problem is, there are nonclosed prime ideals whose closure is a maximal ideal. So, it makes a difference whether you take it for a Banach algebra or for an abstract algebra. In fact, the statement of Corollary is correct, it is the proof which is not. – Alex Gavrilov Jun 26 '14 at 04:33
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    I would like to know more generally what happens if you take the Coquand-Lombardi characterization as a definition and try to develop the basics of dimension theory from there. – Neil Strickland Jun 26 '14 at 14:48
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    @Alex: Please consider double-checking your claims concerning counterexamples, gaps and incomplete proofs. Especially in answers which don't address the question at all. – Martin Brandenburg Jun 26 '14 at 15:43
  • @Neil Strickland: This is an interesting question, but I do not think anybody can answer it, before someone tried to do this. – Alex Gavrilov Jun 27 '14 at 02:40
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    @Martin Brandenburg: Well, I was wrong again. Sorry about bothering you. Anyway, I think you asked a good question. I upvoted it. – Alex Gavrilov Jun 27 '14 at 14:01
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    @MartinBrandenburg: are there known non-Noetherian examples where $\text{dim}(R[T])>\text{dim}(R)+1$? – Neil Strickland Jun 30 '14 at 20:16
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    @NeilStrickland: The following example can be found in Hutchins, Examples of Commutative Rings, Example 27: Let $k$ be a field and let $R = k(y)[[x]] \times_{k(y)} k$ the ring of those power series in $x$ with coefficients in $k(y)$ resp. $k$ for the constant term. Then $\dim(R)=1$ and $\dim(R[T])=3$. I've also read that actually every number between $\dim(R)+1$ and $2 \dim(R)+1$ may appear as $\dim(R[T])$. – Martin Brandenburg Aug 04 '14 at 14:13
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    @Neil: What Martin read (every number between the bound appers) you can also read, namely in A. Seidenberg, On the dimension theory of rings (II), Pacific J. Math. 4 (1954), 603-614. – Fred Rohrer Aug 25 '14 at 21:21
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    Anything wrong with the fact that all polynomials over the trivial ring are 0? IMHO 1 ≠ 0 must be required explicitly. – Incnis Mrsi Dec 20 '14 at 12:57
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    The elementary characterization is also very useful in a topos-internal context. For example, a scheme $X$ is of dimension $\leq n$ if and only if, from the internal perspective of the little Zariski topos $\mathrm{Sh}(X)$, the (then plain old) ring $\mathcal{O}_X$ is of Krull dimension $\leq n$. See Proposition 3.13 of these sketchy notes. – Ingo Blechschmidt Feb 06 '15 at 15:51
  • @Ingo: Thank you, this is one more piece of motivation to read your notes! (I've already started.) – Martin Brandenburg Feb 12 '15 at 23:32
  • Can some moderator make this question CW, please? – Martin Brandenburg May 23 '21 at 08:42
  • @IngoBlechschmidt Let me ask a philosophical question about your characterization. It seems to me that the Krull dimension is a pure topological invariant, namely, completely determined by the Zariski locale (or topos), so the datum of the structure sheaf seems to be redundant? – Z. M Dec 10 '22 at 09:24
  • Coquand--Lombardi is constructive. Do you assume the axiom of choice for the "elementary" proof you want? – Z. M Dec 10 '22 at 09:26
  • Axiom of choice and LEM are OK for me. I just would like to know if there is a proof which doesn't take the detour with prime ideals and non-trivial results from commutative algebra, and instead uses the Coquand-Lombardi characterization. – Martin Brandenburg Dec 10 '22 at 10:46
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    Let me mention that, in the book by Lombardi–Quitté (available on arXiv), §XIII.8, they showed (constructively) that the valuative dimension $\dim_vR[T]$ is $1+\dim_vR$, and if the Krull dimension and the valuative dimension agree for $R$, then they also agree for $R[T]$. – Z. M Dec 10 '22 at 15:32
  • @Z.M I think you are right; however, I still wanted to find internal characterization of dimension, in order to be able to carry out as much work as possible internally to the (little or big) Zariski topos. – Ingo Blechschmidt Dec 16 '22 at 08:24
  • @IngoBlechschmidt Is the homotopy dimension of a topos internal? I am also slightly skeptical about the moral correctness of Zariski toplogy, instead of its Hochster dual. Compare with Lombarbi–Mahboubi, Valuative lattices and spectra §3.1. – Z. M Dec 16 '22 at 14:16
  • Seven answers, all deleted. – Gerry Myerson Nov 23 '23 at 22:47
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    I am not sure why people are editing my question without actually improving it, it seems they just want to apply their way of writing. I have rolled back two times now. Please leave the question as it is. – Martin Brandenburg Nov 25 '23 at 09:00
  • @MartinBrandenbug. But you accepted my edit of your intro verbatim. Previously, you said much more, like "This is not the place to advertise your favorite..." and "will probably be deleted by moderators." And then everyone's answers miraculously get flagged for deletion. It came across as browbeating. How are you to know what moderators will probably do? – Jesse Elliott Nov 29 '23 at 10:07
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    @JesseElliott They don't get "miraculously" flagged. Some people (myself included) voted to delete answers that insisted on addressing the question they wished Martin had asked, and then doubling down when Martin said "that's not what I'm looking for". – Yemon Choi Nov 29 '23 at 22:52
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    @JesseElliott I checked, and yours is actually the only deleted answer that avoids using prime ideals. 3 are nonsense/spam, 2 give variants of the standard proof using prime ideals, 1 is a comment too long to fit in the comment box that turned out to contain a mistake and was probably deleted for that reason. – Will Sawin Nov 30 '23 at 22:04

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