Lemma 37.47.2. Let f : X \to S be a morphism. Let x \in X and set s = f(x). Assume that f is locally of finite type and that n = \dim _ x(X_ s). Then there exists a commutative diagram
\xymatrix{ X \ar[dd] & X' \ar[l]^ g \ar[d]^\pi & x \ar@{|->}[dd] & x' \ar@{|->}[l] \ar@{|->}[d] \\ & Y' \ar[d]^ h & & y' \ar@{|->}[d] \\ S & S' \ar[l]_ e & s & s' \ar@{|->}[l] }
and a point x' \in X' with g(x') = x such that with y' = \pi (x'), s' = h(y') we have
h : Y' \to S' is smooth of relative dimension n,
all fibres of Y' \to S' are geometrically integral,
g : (X', x') \to (X, x) is an elementary étale neighbourhood,
\pi is finite, and \pi ^{-1}(\{ y'\} ) = \{ x'\} ,
\kappa (y') is a purely transcendental extension of \kappa (s'), and
e : (S', s') \to (S, s) is an elementary étale neighbourhood.
Moreover, if f is locally of finite presentation, then \pi is of finite presentation.
Proof.
The question is local on S, hence we may replace S by an affine open neighbourhood of s. Next, we apply Lemma 37.47.1 to get a commutative diagram
\xymatrix{ X \ar[dd] & X' \ar[l]^ g \ar[d]^\pi & x \ar@{|->}[dd] & x' \ar@{|->}[l] \ar@{|->}[d] \\ & Y \ar[d]^ h & & y \ar@{|->}[d] \\ S \ar@{=}[r] & S & s & s \ar@{=}[l] }
where h is smooth of relative dimension n and \kappa (y) is a purely transcendental extension of \kappa (s). Since the question is local on X also, we may replace Y by an affine neighbourhood of y (and X' by the inverse image of this under \pi ). As S is affine this guarantees that Y \to S is quasi-compact, separated and smooth, in particular of finite presentation. Let T be the connected component of Y_ s containing y. As Y_ s is Noetherian we see that T is open. We also see that T is geometrically connected over \kappa (s) by Varieties, Lemma 33.7.14. Since T is also smooth over \kappa (s) it is geometrically normal, see Varieties, Lemma 33.25.4. We conclude that T is geometrically irreducible over \kappa (s) (as a connected Noetherian normal scheme is irreducible, see Properties, Lemma 28.7.6). Finally, note that the smooth morphism h is normal by Lemma 37.20.3. At this point we have verified all assumption of Lemma 37.46.4 hold for the morphism h : Y \to S and open T \subset Y_ s. As a result of applying Lemma 37.46.4 we obtain e : S' \to S, s' \in S', Y' as in the commutative diagram
\xymatrix{ X \ar[dd] & X' \ar[l]^ g \ar[d]^\pi & X' \times _ Y Y' \ar[l] \ar[d] & x \ar@{|->}[dd] & x' \ar@{|->}[l] \ar@{|->}[d] & (x', s') \ar@{|->}[l] \ar@{|->}[d] \\ & Y \ar[d]^ h & Y' \ar[d] \ar[l] & & y \ar@{|->}[d] & (y, s') \ar@{|->}[l] \ar@{|->}[d] \\ S \ar@{=}[r] & S & S' \ar[l]_ e & s & s \ar@{=}[l] & s' \ar@{|->}[l] }
where e : (S', s') \to (S, s) is an elementary étale neighbourhood, and where Y' \subset Y_{S'} is an open neighbourhood all of whose fibres over S' are geometrically irreducible, such that Y'_{s'} = T via the identification Y_ s = Y_{S', s'}. Let (y, s') \in Y' be the point corresponding to y \in T; this is also the unique point of Y \times _ S S' lying over y with residue field equal to \kappa (y) which maps to s' in S'. Similarly, let (x', s') \in X' \times _ Y Y' \subset X' \times _ S S' be the unique point over x' with residue field equal to \kappa (x') lying over s'. Then the outer part of this diagram is a solution to the problem posed in the lemma. Some minor details omitted.
\square
Comments (1)
Comment #1127 by Simon Pepin Lehalleur on