Example 26.20.7. Let k be a field. Consider the structure morphism p : \mathbf{P}^1_ k \to \mathop{\mathrm{Spec}}(k) of the projective line over k, see Example 26.14.4. Let us use the valuative criterion above to prove that p is universally closed. By construction \mathbf{P}^1_ k is covered by two affine opens and hence p is quasi-compact. Let a commutative diagram
be given, where A is a valuation ring and K is its field of fractions. Recall that \mathbf{P}^1_ k is gotten by glueing \mathop{\mathrm{Spec}}(k[x]) to \mathop{\mathrm{Spec}}(k[y]) by glueing D(x) to D(y) via x = y^{-1} (or more symmetrically xy = 1). To show there is a morphism \mathop{\mathrm{Spec}}(A) \to \mathbf{P}^1_ k fitting diagonally into the diagram above we may assume that \xi maps into the open \mathop{\mathrm{Spec}}(k[x]) (by symmetry). This gives the following commutative diagram of rings
By Algebra, Lemma 10.50.4 we see that either \xi ^\sharp (x) \in A or \xi ^\sharp (x)^{-1} \in A. In the first case we get a ring map
fitting into the diagram of rings above, and we win. In the second case we see that we get a ring map
This gives a morphism \mathop{\mathrm{Spec}}(A) \to \mathop{\mathrm{Spec}}(k[y]) \to \mathbf{P}^1_ k which fits diagonally into the initial commutative diagram of this example (check omitted).
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