The Stacks project

Proof. We first use absolute Noetherian approximation to reduce to the case of schemes of finite type over $\mathbf{Z}$. The question is local on $X$ and $S$. Hence we may assume that $X$ and $S$ are affine. Then we can write $S = \mathop{\mathrm{Spec}}(R)$ and write $R$ as a filtered colimit $R = \mathop{\mathrm{colim}}\nolimits R_ i$ of finite type $\mathbf{Z}$-algebras. Using Limits, Lemma 32.10.1 we can find an $i$ and a morphism $f_ i : X_ i \to \mathop{\mathrm{Spec}}(R_ i)$ whose base change to $S$ is $f$. After increasing $i$ we may assume that $f_ i$ is at-worst-nodal of relative dimension $1$, see Lemma 53.20.10. The image $x_ i \in X_ i$ of $x$ will be a singular point of its fibre, for example because the formation of $\text{Sing}(f)$ commutes with base change (Divisors, Lemma 31.10.1). If we can prove the lemma for $f_ i : X_ i \to S_ i$ and $x_ i$, then the lemma follows for $f : X \to S$ by base change. Thus we reduce to the case studied in the next paragraph.

Assume $S$ is of finite type over $\mathbf{Z}$. Let $s \in S$ be the image of $x$. Recall that $\kappa (x)$ is a finite separable extension of $\kappa (s)$, for example because $\text{Sing}(f) \to S$ is unramified or because $x$ is a node of the fibre $X_ s$ and we can apply Lemma 53.19.7. Furthermore, let $\kappa '/\kappa (x)$ be the degree $2$ separable algebra associated to $\mathcal{O}_{X_ s, x}$ in Remark 53.19.8. By More on Morphisms, Lemma 37.35.2 we can choose an étale neighbourhood $(V, v) \to (S, s)$ such that the extension $\kappa (v)/\kappa (s)$ realizes either the extension $\kappa (x)/\kappa (s)$ in case $\kappa ' \cong \kappa (x) \times \kappa (x)$ or the extension $\kappa '/\kappa (s)$ if $\kappa '$ is a field. After replacing $X$ by $X \times _ S V$ and $S$ by $V$ we reduce to the situation described in the next paragraph.

Assume $S$ is of finite type over $\mathbf{Z}$ and $x \in X_ s$ is a split node, see Definition 53.19.10. By Lemma 53.20.11 we see that there exists an $\mathcal{O}_{S, s}$-algebra isomorphism

for some $h \in \mathfrak m_ s^\wedge \subset \mathcal{O}_{S, s}^\wedge$. In other words, if we consider the homomorphism

sending $a$ to $h$, then there exists an $\mathcal{O}_{S, s}$-algebra isomorphism

where

and $y_\sigma$ is the point of $Y_\sigma$ lying over the closed point of $\mathop{\mathrm{Spec}}(\mathcal{O}_{S, s}^\wedge )$ and having coordinates $u, v$ equal to zero. Since $\mathcal{O}_{S, s}$ is a G-ring by More on Algebra, Proposition 15.50.12 we may apply More on Morphisms, Lemma 37.39.3 to conclude. $\square$