Lemma 29.42.5. Let S be a scheme. Let X, Y be schemes over S. Let x \in X. Let U \subset X be an open and let f : U \to Y be a morphism over S. Assume
x is in the closure of U,
X is reduced with finitely many irreducible components or X is Noetherian,
\mathcal{O}_{X, x} is a valuation ring,
Y \to S is proper
Then there exists an open U \subset U' \subset X containing x and an S-morphism f' : U' \to Y extending f.
Proof.
It is harmless to replace X by an open neighbourhood of x in X (small detail omitted). By Properties, Lemma 28.29.8 we may assume X is affine with \Gamma (X, \mathcal{O}_ X) \subset \mathcal{O}_{X, x}. In particular X is integral with a unique generic point \xi whose residue field is the fraction field K of the valuation ring \mathcal{O}_{X, x}. Since x is in the closure of U we see that U is not empty, hence U contains \xi . Thus by the valuative criterion of properness (Lemma 29.42.1) there is a morphism t : \mathop{\mathrm{Spec}}(\mathcal{O}_{X, x}) \to Y fitting into a commutative diagram
\xymatrix{ \mathop{\mathrm{Spec}}(K) \ar[d]_\xi \ar[r] & \mathop{\mathrm{Spec}}(\mathcal{O}_{X, x}) \ar[d]_ t \\ U \ar[r]^ f & Y }
of morphisms of schemes over S. Applying Lemma 29.42.4 with y = t(x) and \varphi = t^\sharp _ x we obtain an open neighbourhood V \subset X of x and a morphism g : V \to Y over S which sends x to y and such that g^\sharp _ x = t^\sharp _ x. As Y \to S is separated, the equalizer E of f|_{U \cap V} and g|_{U \cap V} is a closed subscheme of U \cap V, see Schemes, Lemma 26.21.5. Since f and g determine the same morphism \mathop{\mathrm{Spec}}(K) \to Y by construction we see that E contains the generic point of the integral scheme U \cap V. Hence E = U \cap V and we conclude that f and g glue to a morphism U' = U \cup V \to Y as desired.
\square
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