Gluing K-morphisms into (Kⁿ, 𝒜_{Kⁿ}) along an open cover #
A K-local-ringed space Z covered by opens U i, with K-morphisms
h i : Z|U i ⟶ (Kⁿ, 𝒜_{Kⁿ}) that agree on the overlaps — the same base point, and the same germ
of the pullback of every function of 𝒜_{Kⁿ} at every point of U i ∩ U j — has a K-morphism
Z ⟶ (Kⁿ, 𝒜_{Kⁿ}) (glueHomAffine) which agrees with h i on U i in the same two senses:
on base points (toFun_glueHomAffine_eq) and on the germs of pulled-back sections
(germ_pullbackΓ_glueHomAffine); the equation ofRestrict Z (U i) ≫ glueHomAffine … = h i
itself is not proved here. The agreement hypotheses are
stated on germs, so that everything is decided by TopCat.Presheaf.section_ext and the sheaf
property of 𝒪_Z; in the application (a morphism X ⟶ (Kⁿ, 𝒜_{Kⁿ}) of an analytic K-space
from n global sections, Hironaka/AnalyticSpace/HomOfSections.lean) the agreement comes from the
uniqueness ringHom_ext_of_coord on the stalks. This is the gluing step in the construction of
Hironaka's extended coordination h' [Hir64, Ch. 0, §1, p. 120]. Elementary sheaf theory; not
in the sources.
Construction. The base map is z ↦ (h i).base z for any i with z ∈ U i (well defined and
continuous because it is so on each U i). For an open V ⊆ Kⁿ and F ∈ 𝒜(V), the sections
(h i)^*F over U i ∩ b⁻¹(V) are compatible (equal germs, hence equal by section_ext), and
glue to a section over b⁻¹(V) (existsUnique_gluing'); uniqueness of the gluing gives the
naturality and the ring-homomorphism axioms of the sheaf map. The stalk map at z ∈ U i is the
stalk map of h i (through the isomorphism of the stalk of Z|U i with that of Z), hence
local, and the constants pull back to constants because they do so for each h i.
The image of an open of an open subspace #
The image in Z of an open T of the open subspace Z|V: the functor of the open embedding
V → Z. Sections of 𝒪_{Z|V} over T are, by definition of the restriction, sections of 𝒪_Z
over imgOpens Z V T.
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The image of the whole open subspace Z|W is contained in W.
The glued base map #
An index of the cover containing z.
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The glued base map z ↦ (h i).base z, z ∈ U i.
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The glued base map as a morphism of TopCat.
Equations
- Z.glueBaseHom U hU h hbase = TopCat.ofHom { toFun := Z.glueBase U hU h, continuous_toFun := ⋯ }
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The glued sections #
The preimage of V ⊆ Kⁿ under the glued base map.
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- Z.preV U hU h hbase V = (TopologicalSpace.Opens.map (Z.glueBaseHom U hU h hbase)).obj V
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The piece U i ∩ b⁻¹(V) of the preimage, as the image of the preimage of V under h i.
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The pullback of F ∈ 𝒜(V) along h i, as a section of 𝒪_Z over U i ∩ b⁻¹(V).
Equations
- Z.locSec U h i V F = (CategoryTheory.ConcreteCategory.hom ((↑(h i)).c.app (Opposite.op V))) F
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The glued pullback of F ∈ 𝒜(V): the section of 𝒪_Z over b⁻¹(V) restricting to (h i)^*F
on every U i ∩ b⁻¹(V).
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The germ of the glued section at z ∈ U i is the germ of (h i)^*F.
Two sections over b⁻¹(V) with the same germs as the local pullbacks are equal: the
uniqueness in existsUnique_glueSec, at the level of germs.
The glued sheaf map #
The component at V of the glued sheaf map, as a ring homomorphism F ↦ glueSec F.
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- One or more equations did not get rendered due to their size.
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The sheaf map 𝒜_{Kⁿ} ⟶ b_* 𝒪_Z of the glued morphism: F ↦ glueSec F.
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- One or more equations did not get rendered due to their size.
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The glued morphism #
The glued morphism of presheafed spaces.
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The stalk map of the glued morphism on a germ: the germ of the local pullback (h i)^*F.
The isomorphism of the stalk of Z at z ∈ W with the stalk of Z|W on a germ: a section of
𝒪_Z over the image of an open T of Z|W is a section of 𝒪_{Z|W} over T, with the
corresponding germ (Mathlib's restrictStalkIso_inv_eq_germ).
The stalk maps of the glued morphism are local: a germ whose glued pullback is a unit has a
nonzero value, because the pullback along h i is a unit and h i is a K-morphism.
The glued K-morphism Z ⟶ (Kⁿ, 𝒜_{Kⁿ}) of compatible morphisms h i on an open cover
of Z.
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The germ at z ∈ U i of the pullback of a global section of 𝒜_{Kⁿ} along the glued
morphism is the germ of its pullback along h i.