The Stacks project

Lemma 10.135.13. Let $R$ be a ring. Let $S = R[x_1, \ldots , x_ n]/(f_1, \ldots , f_ c)$ be a relative global complete intersection (Definition 10.135.5). For every prime $\mathfrak q$ of $S$, let $\mathfrak q'$ denote the corresponding prime of $R[x_1, \ldots , x_ n]$. Then

  1. $f_1, \ldots , f_ c$ is a regular sequence in the local ring $R[x_1, \ldots , x_ n]_{\mathfrak q'}$,

  2. each of the rings $R[x_1, \ldots , x_ n]_{\mathfrak q'}/(f_1, \ldots , f_ i)$ is flat over $R$, and

  3. the $S$-module $(f_1, \ldots , f_ c)/(f_1, \ldots , f_ c)^2$ is free with basis given by the elements $f_ i \bmod (f_1, \ldots , f_ c)^2$.

Proof. First, by Lemma 10.68.2, part (3) follows from part (1). Parts (1) and (2) immediately reduce to the Noetherian case by Lemma 10.135.12 (some minor details omitted). Assume $R$ is Noetherian. Let $\mathfrak p = R \cap \mathfrak q'$. By Lemma 10.134.4 for example we see that $f_1, \ldots , f_ c$ form a regular sequence in the local ring $R[x_1, \ldots , x_ n]_{\mathfrak q'} \otimes _ R \kappa (\mathfrak p)$. Moreover, the local ring $R[x_1, \ldots , x_ n]_{\mathfrak q'}$ is flat over $R_{\mathfrak p}$. Since $R$, and hence $R[x_1, \ldots , x_ n]_{\mathfrak q'}$ is Noetherian we may apply Lemma 10.98.3 to conclude. $\square$


Comments (4)

Comment #3254 by Dario WeiƟmann on

We could define . It is clear from the context, but still.

Comment #4721 by comment_bot on

A pedantic comment: I suggest adding "such that every nonempty -fiber of has dimension " to the second sentence of the statement. This would make the statement less ambiguous: theoretically, in that sentence it is not clear whether we are choosing a presentation as in the definition of a "global complete intersection" or whether we are choosing an arbitrary presentation for the -algebra (knowing that happens to be a global complete intersection, as witnessed by some other presentation).

The same pedantic comment applies to other statements in this section.

Comment #4811 by on

Tried to improve the wording of these lemmas. See changes here.

There are also:

  • 2 comment(s) on Section 10.135: Syntomic morphisms

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