Proposition 38.10.3. Let f : X \to S be a morphism of schemes. Let \mathcal{F} be a quasi-coherent sheaf on X. Let x \in X with image s \in S. Assume that
f is locally of finite presentation,
\mathcal{F} is of finite type, and
\mathcal{F} is flat at x over S.
Then there exists an elementary étale neighbourhood (S', s') \to (S, s) and an open subscheme
V \subset X \times _ S \mathop{\mathrm{Spec}}(\mathcal{O}_{S', s'})
which contains the unique point of X \times _ S \mathop{\mathrm{Spec}}(\mathcal{O}_{S', s'}) mapping to x such that the pullback of \mathcal{F} to V is an \mathcal{O}_ V-module of finite presentation and flat over \mathcal{O}_{S', s'}.
First proof.
This proof is longer but does not use the existence of a complete dévissage. The problem is local around x and s, hence we may assume that X and S are affine. During the proof we will finitely many times replace S by an elementary étale neighbourhood of (S, s). The goal is then to find (after such a replacement) an open V \subset X \times _ S \mathop{\mathrm{Spec}}(\mathcal{O}_{S, s}) containing x such that \mathcal{F}|_ V is flat over S and finitely presented. Of course we may also replace S by \mathop{\mathrm{Spec}}(\mathcal{O}_{S, s}) at any point of the proof, i.e., we may assume S is a local scheme. We will prove the proposition by induction on the integer n = \dim _ x(\text{Supp}(\mathcal{F}_ s)).
We can choose
elementary étale neighbourhoods g : (X', x') \to (X, x), e : (S', s') \to (S, s),
a commutative diagram
\xymatrix{ X \ar[dd]_ f & X' \ar[dd] \ar[l]^ g & Z' \ar[l]^ i \ar[d]^\pi \\ & & Y' \ar[d]^ h \\ S & S' \ar[l]_ e & S' \ar@{=}[l] }
a point z' \in Z' with i(z') = x', y' = \pi (z'), h(y') = s',
a finite type quasi-coherent \mathcal{O}_{Z'}-module \mathcal{G},
as in Lemma 38.3.2. We are going to replace S by \mathop{\mathrm{Spec}}(\mathcal{O}_{S', s'}), see remarks in first paragraph of the proof. Consider the diagram
\xymatrix{ X_{\mathcal{O}_{S', s'}} \ar[ddr]_ f & X'_{\mathcal{O}_{S', s'}} \ar[dd] \ar[l]^ g & Z'_{\mathcal{O}_{S', s'}} \ar[l]^ i \ar[d]^\pi \\ & & Y'_{\mathcal{O}_{S', s'}} \ar[dl]^ h \\ & \mathop{\mathrm{Spec}}(\mathcal{O}_{S', s'}) }
Here we have base changed the schemes X', Z', Y' over S' via \mathop{\mathrm{Spec}}(\mathcal{O}_{S', s'}) \to S' and the scheme X over S via \mathop{\mathrm{Spec}}(\mathcal{O}_{S', s'}) \to S. It is still the case that g is étale, see Lemma 38.2.2. After replacing X by X_{\mathcal{O}_{S', s'}}, X' by X'_{\mathcal{O}_{S', s'}}, Z' by Z'_{\mathcal{O}_{S', s'}}, and Y' by Y'_{\mathcal{O}_{S', s'}} we may assume we have a diagram as Lemma 38.3.2 where in addition S = S' is a local scheme with closed point s. By Lemmas 38.3.3 and 38.3.4 the result for Y' \to S, the sheaf \pi _*\mathcal{G}, and the point y' implies the result for X \to S, \mathcal{F} and x. Hence we may assume that S is local and X \to S is a smooth morphism of affines with geometrically irreducible fibres of dimension n.
The base case of the induction: n = 0. As X \to S is smooth with geometrically irreducible fibres of dimension 0 we see that X \to S is an open immersion, see Descent, Lemma 35.25.2. As S is local and the closed point is in the image of X \to S we conclude that X = S. Thus we see that \mathcal{F} corresponds to a finite flat \mathcal{O}_{S, s} module. In this case the result follows from Algebra, Lemma 10.78.5 which tells us that \mathcal{F} is in fact finite free.
The induction step. Assume the result holds whenever the dimension of the support in the closed fibre is < n. Write S = \mathop{\mathrm{Spec}}(A), X = \mathop{\mathrm{Spec}}(B) and \mathcal{F} = \widetilde{N} for some B-module N. Note that A is a local ring; denote its maximal ideal \mathfrak m. Then \mathfrak p = \mathfrak mB is the unique minimal prime lying over \mathfrak m as X \to S has geometrically irreducible fibres. Finally, let \mathfrak q \subset B be the prime corresponding to x. By Lemma 38.10.1 we can choose a map
\alpha : B^{\oplus r} \to N
such that \kappa (\mathfrak p)^{\oplus r} \to N \otimes _ B \kappa (\mathfrak p) is an isomorphism. Moreover, as N_{\mathfrak q} is A-flat the lemma also shows that \alpha is injective and that \mathop{\mathrm{Coker}}(\alpha )_{\mathfrak q} is A-flat. Set Q = \mathop{\mathrm{Coker}}(\alpha ). Note that the support of Q/\mathfrak mQ does not contain \mathfrak p. Hence it is certainly the case that \dim _{\mathfrak q}(\text{Supp}(Q/\mathfrak mQ)) < n. Combining everything we know about Q we see that the induction hypothesis applies to Q. It follows that there exists an elementary étale morphism (S', s) \to (S, s) such that the conclusion holds for Q \otimes _ A A' over B \otimes _ A A' where A' = \mathcal{O}_{S', s'}. After replacing A by A' we have an exact sequence
0 \to B^{\oplus r} \to N \to Q \to 0
(here we use that \alpha is injective as mentioned above) of finite B-modules and we also get an element g \in B, g \not\in \mathfrak q such that Q_ g is finitely presented over B_ g and flat over A. Since localization is exact we see that
0 \to B_ g^{\oplus r} \to N_ g \to Q_ g \to 0
is still exact. As B_ g and Q_ g are flat over A we conclude that N_ g is flat over A, see Algebra, Lemma 10.39.13, and as B_ g and Q_ g are finitely presented over B_ g the same holds for N_ g, see Algebra, Lemma 10.5.3.
\square
Second proof.
We apply Proposition 38.5.7 to find a commutative diagram
\xymatrix{ (X, x) \ar[d] & (X', x') \ar[l]^ g \ar[d] \\ (S, s) & (S', s') \ar[l] }
of pointed schemes such that the horizontal arrows are elementary étale neighbourhoods and such that g^*\mathcal{F}/X'/S' has a complete dévissage at x. (In particular S' and X' are affine.) By Morphisms, Lemma 29.25.13 we see that g^*\mathcal{F} is flat at x' over S and by Lemma 38.2.3 we see that it is flat at x' over S'. Via Remark 38.6.5 we deduce that
\Gamma (X', g^*\mathcal{F})/ \Gamma (X', \mathcal{O}_{X'})/ \Gamma (S', \mathcal{O}_{S'})
has a complete dévissage at the prime of \Gamma (X', \mathcal{O}_{X'}) corresponding to x'. We may base change this complete dévissage to the local ring \mathcal{O}_{S', s'} of \Gamma (S', \mathcal{O}_{S'}) at the prime corresponding to s'. Thus Lemma 38.10.2 implies that
\Gamma (X', \mathcal{F}') \otimes _{\Gamma (S', \mathcal{O}_{S'})} \mathcal{O}_{S', s'}
is flat over \mathcal{O}_{S', s'} and of finite presentation over \Gamma (X', \mathcal{O}_{X'}) \otimes _{\Gamma (S', \mathcal{O}_{S'})} \mathcal{O}_{S', s'}. In other words, the restriction of \mathcal{F} to X' \times _{S'} \mathop{\mathrm{Spec}}(\mathcal{O}_{S', s'}) is of finite presentation and flat over \mathcal{O}_{S', s'}. Since the morphism X' \times _{S'} \mathop{\mathrm{Spec}}(\mathcal{O}_{S', s'}) \to X \times _ S \mathop{\mathrm{Spec}}(\mathcal{O}_{S', s'}) is étale (Lemma 38.2.2) its image V \subset X \times _ S \mathop{\mathrm{Spec}}(\mathcal{O}_{S', s'}) is an open subscheme, and by étale descent the restriction of \mathcal{F} to V is of finite presentation and flat over \mathcal{O}_{S', s'}. (Results used: Morphisms, Lemma 29.36.13, Descent, Lemma 35.7.3, and Morphisms, Lemma 29.25.13.)
\square
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