
Lemma 10.134.11. Let $R$ be a ring. Let $S = R[x_1, \ldots , x_ n]/(f_1, \ldots , f_ c)$. We will find $h \in R[x_1, \ldots , x_ n]$ which maps to $g \in S$ such that

$S_ g = R[x_1, \ldots , x_ n, x_{n + 1}]/(f_1, \ldots , f_ c, hx_{n + 1} - 1)$

is a relative global complete intersection over $R$ in each of the following cases:

1. Let $I \subset R$ be an ideal. If the fibres of $\mathop{\mathrm{Spec}}(S/IS) \to \mathop{\mathrm{Spec}}(R/I)$ have dimension $n - c$, then we can find $(h, g)$ as above such that $g$ maps to $1 \in S/IS$.

2. Let $\mathfrak p \subset R$ be a prime. If $\dim (S \otimes _ R \kappa (\mathfrak p)) = n - c$, then we can find $(h, g)$ as above such that $g$ maps to a unit of $S \otimes _ R \kappa (\mathfrak p)$.

3. Let $\mathfrak q \subset S$ be a prime lying over $\mathfrak p \subset R$. If $\dim _{\mathfrak q}(S/R) = n - c$, then we can find $(h, g)$ as above such that $g \not\in \mathfrak q$.

Proof. Ad (1). By Lemma 10.124.6 there exists an open subset $W \subset \mathop{\mathrm{Spec}}(S)$ containing $V(IS)$ such that all fibres of $W \to \mathop{\mathrm{Spec}}(R)$ have dimension $\leq n - c$. Say $W = \mathop{\mathrm{Spec}}(S) \setminus V(J)$. Then $V(J) \cap V(IS) = \emptyset$ hence we can find a $g \in J$ which maps to $1 \in S/IS$. Let $h \in R[x_1, \ldots , x_ n]$ be any preimage of $g$.

Ad (2). By Lemma 10.124.6 there exists an open subset $W \subset \mathop{\mathrm{Spec}}(S)$ containing $\mathop{\mathrm{Spec}}(S \otimes _ R \kappa (\mathfrak p))$ such that all fibres of $W \to \mathop{\mathrm{Spec}}(R)$ have dimension $\leq n - c$. Say $W = \mathop{\mathrm{Spec}}(S) \setminus V(J)$. Then $V(J \cdot S \otimes _ R \kappa (\mathfrak p)) = \emptyset$. Hence we can find a $g \in J$ which maps to a unit in $S \otimes _ R \kappa (\mathfrak p)$ (details omitted). Let $h \in R[x_1, \ldots , x_ n]$ be any preimage of $g$.

Ad (3). By Lemma 10.124.6 there exists a $g \in S$, $g \not\in \mathfrak q$ such that all nonempty fibres of $R \to S_ g$ have dimension $\leq n - c$. Let $h \in R[x_1, \ldots , x_ n]$ be any element that maps to $g$. $\square$

Comment #717 by Keenan Kidwell on

In (3), "then find" should be "then we can find."

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