
[II Proposition 7.2.3, EGA]

Proof. By assumption (1), Proposition 25.20.6, and Lemma 25.10.4 we see that it suffices to prove the morphism $\Delta _{X/S} : X \to X \times _ S X$ satisfies the existence part of the valuative criterion. Let a solid commutative diagram

$\xymatrix{ \mathop{\mathrm{Spec}}(K) \ar[r] \ar[d] & X \ar[d] \\ \mathop{\mathrm{Spec}}(A) \ar[r] \ar@{-->}[ru] & X \times _ S X }$

be given. The lower right arrow corresponds to a pair of morphisms $a, b : \mathop{\mathrm{Spec}}(A) \to X$ over $S$. By (2) we see that $a = b$. Hence using $a$ as the dotted arrow works. $\square$

Comment #1725 by Keenan Kidwell on

Maybe it would be good to add, in addition to the reference to 01KF, a reference (in the same place) to 01IQ, since the valuative criterion shows (in particular) that the diagonal is closed, while 01IQ says that this closedness is enough to conclude that the diagonal is a closed immersion.

Comment #2706 by Ariyan Javanpeykar on

A reference for the valuative criterion of separatedness: EGA II, Proposition 7.2.3

Comment #3814 by Kestutis Cesnavicius on

Same comment as for https://stacks.math.columbia.edu/tag/01KE

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