61.3 Local isomorphisms
We start with a definition.
Definition 61.3.1. Let \varphi : A \to B be a ring map.
We say A \to B is a local isomorphism if for every prime \mathfrak q \subset B there exists a g \in B, g \not\in \mathfrak q such that A \to B_ g induces an open immersion \mathop{\mathrm{Spec}}(B_ g) \to \mathop{\mathrm{Spec}}(A).
We say A \to B identifies local rings if for every prime \mathfrak q \subset B the canonical map A_{\varphi ^{-1}(\mathfrak q)} \to B_\mathfrak q is an isomorphism.
We list some elementary properties.
Lemma 61.3.2. Let A \to B and A \to A' be ring maps. Let B' = B \otimes _ A A' be the base change of B.
If A \to B is a local isomorphism, then A' \to B' is a local isomorphism.
If A \to B identifies local rings, then A' \to B' identifies local rings.
Proof.
Omitted.
\square
Lemma 61.3.3. Let A \to B and B \to C be ring maps.
If A \to B and B \to C are local isomorphisms, then A \to C is a local isomorphism.
If A \to B and B \to C identify local rings, then A \to C identifies local rings.
Proof.
Omitted.
\square
Lemma 61.3.4. Let A be a ring. Let B \to C be an A-algebra homomorphism.
If A \to B and A \to C are local isomorphisms, then B \to C is a local isomorphism.
If A \to B and A \to C identify local rings, then B \to C identifies local rings.
Proof.
Omitted.
\square
Lemma 61.3.5. Let A \to B be a local isomorphism. Then
A \to B is étale,
A \to B identifies local rings,
A \to B is quasi-finite.
Proof.
Omitted.
\square
Lemma 61.3.6. Let A \to B be a local isomorphism. Then there exist n \geq 0, g_1, \ldots , g_ n \in B, f_1, \ldots , f_ n \in A such that (g_1, \ldots , g_ n) = B and A_{f_ i} \cong B_{g_ i}.
Proof.
Omitted.
\square
Lemma 61.3.7. Let p : (Y, \mathcal{O}_ Y) \to (X, \mathcal{O}_ X) and q : (Z, \mathcal{O}_ Z) \to (X, \mathcal{O}_ X) be morphisms of locally ringed spaces. If \mathcal{O}_ Y = p^{-1}\mathcal{O}_ X, then
\mathop{\mathrm{Mor}}\nolimits _{\text{LRS}/(X, \mathcal{O}_ X)}((Z, \mathcal{O}_ Z), (Y, \mathcal{O}_ Y)) \longrightarrow \mathop{\mathrm{Mor}}\nolimits _{\textit{Top}/X}(Z, Y),\quad (f, f^\sharp ) \longmapsto f
is bijective. Here \text{LRS}/(X, \mathcal{O}_ X) is the category of locally ringed spaces over X and \textit{Top}/X is the category of topological spaces over X.
Proof.
This is immediate from the definitions.
\square
Lemma 61.3.8. Let A be a ring. Set X = \mathop{\mathrm{Spec}}(A). The functor
B \longmapsto \mathop{\mathrm{Spec}}(B)
from the category of A-algebras B such that A \to B identifies local rings to the category of topological spaces over X is fully faithful.
Proof.
This follows from Lemma 61.3.7 and the fact that if A \to B identifies local rings, then the pullback of the structure sheaf of \mathop{\mathrm{Spec}}(A) via p : \mathop{\mathrm{Spec}}(B) \to \mathop{\mathrm{Spec}}(A) is equal to the structure sheaf of \mathop{\mathrm{Spec}}(B).
\square
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