Fibred products of schemes #
In this file we construct the fibred product of schemes via gluing. We roughly follow [har77] Theorem 3.3.
In particular, the main construction is to show that for an open cover { Uᵢ }
of X
, if there
exist fibred products Uᵢ ×[Z] Y
for each i
, then there exists a fibred product X ×[Z] Y
.
Then, for constructing the fibred product for arbitrary schemes X, Y, Z
, we can use the
construction to reduce to the case where X, Y, Z
are all affine, where fibred products are
constructed via tensor products.
The intersection of Uᵢ ×[Z] Y
and Uⱼ ×[Z] Y
is given by (Uᵢ ×[Z] Y) ×[X] Uⱼ
Equations
- One or more equations did not get rendered due to their size.
Instances For
The canonical transition map (Uᵢ ×[Z] Y) ×[X] Uⱼ ⟶ (Uⱼ ×[Z] Y) ×[X] Uᵢ
given by the fact
that pullbacks are associative and symmetric.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The inclusion map of V i j = (Uᵢ ×[Z] Y) ×[X] Uⱼ ⟶ Uᵢ ×[Z] Y
Equations
- One or more equations did not get rendered due to their size.
Instances For
The map ((Xᵢ ×[Z] Y) ×[X] Xⱼ) ×[Xᵢ ×[Z] Y] ((Xᵢ ×[Z] Y) ×[X] Xₖ)
⟶
((Xⱼ ×[Z] Y) ×[X] Xₖ) ×[Xⱼ ×[Z] Y] ((Xⱼ ×[Z] Y) ×[X] Xᵢ)
needed for gluing
Equations
- One or more equations did not get rendered due to their size.
Instances For
Given Uᵢ ×[Z] Y
, this is the glued fibered product X ×[Z] Y
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The first projection from the glued scheme into X
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The second projection from the glued scheme into Y
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
(Implementation)
The canonical map (s.X ×[X] Uᵢ) ×[s.X] (s.X ×[X] Uⱼ) ⟶ (Uᵢ ×[Z] Y) ×[X] Uⱼ
This is used in gluedLift
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The lifted map s.X ⟶ (gluing 𝒰 f g).glued
in order to show that (gluing 𝒰 f g).glued
is
indeed the pullback.
Given a pullback cone s
, we have the maps s.fst ⁻¹' Uᵢ ⟶ Uᵢ
and
s.fst ⁻¹' Uᵢ ⟶ s.X ⟶ Y
that we may lift to a map s.fst ⁻¹' Uᵢ ⟶ Uᵢ ×[Z] Y
.
to glue these into a map s.X ⟶ Uᵢ ×[Z] Y
, we need to show that the maps agree on
(s.fst ⁻¹' Uᵢ) ×[s.X] (s.fst ⁻¹' Uⱼ) ⟶ Uᵢ ×[Z] Y
. This is achieved by showing that both of these
maps factors through gluedLiftPullbackMap
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
(Implementation)
The canonical map (W ×[X] Uᵢ) ×[W] (Uⱼ ×[Z] Y) ⟶ (Uⱼ ×[Z] Y) ×[X] Uᵢ = V j i
where W
is
the glued fibred product.
This is used in lift_comp_ι
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
We show that the map W ×[X] Uᵢ ⟶ Uᵢ ×[Z] Y ⟶ W
is the first projection, where the
first map is given by the lift of W ×[X] Uᵢ ⟶ Uᵢ
and W ×[X] Uᵢ ⟶ W ⟶ Y
.
It suffices to show that the two map agrees when restricted onto Uⱼ ×[Z] Y
. In this case,
both maps factor through V j i
via pullback_fst_ι_to_V
The canonical isomorphism between W ×[X] Uᵢ
and Uᵢ ×[X] Y
. That is, the preimage of Uᵢ
in
W
along p1
is indeed Uᵢ ×[X] Y
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The glued scheme ((gluing 𝒰 f g).glued
) is indeed the pullback of f
and g
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Equations
- ⋯ = ⋯
Equations
- ⋯ = ⋯
Equations
- ⋯ = ⋯
Equations
- ⋯ = ⋯
Equations
- ⋯ = ⋯
Given an open cover { Xᵢ }
of X
, then X ×[Z] Y
is covered by Xᵢ ×[Z] Y
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Given an open cover { Yᵢ }
of Y
, then X ×[Z] Y
is covered by X ×[Z] Yᵢ
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Given an open cover { Xᵢ }
of X
and an open cover { Yⱼ }
of Y
, then
X ×[Z] Y
is covered by Xᵢ ×[Z] Yⱼ
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
(Implementation). Use openCoverOfBase
instead.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Given an open cover { Zᵢ }
of Z
, then X ×[Z] Y
is covered by Xᵢ ×[Zᵢ] Yᵢ
, where
Xᵢ = X ×[Z] Zᵢ
and Yᵢ = Y ×[Z] Zᵢ
is the preimage of Zᵢ
in X
and Y
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Equations
- ⋯ = ⋯
The isomorphism between the fiber product of two schemes Spec S
and Spec T
over a scheme Spec R
and the Spec
of the tensor product S ⊗[R] T
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The composition of the inverse of the isomorphism pullbackSepcIso R S T
(from the pullback of
Spec S ⟶ Spec R
and Spec T ⟶ Spec R
to Spec (S ⊗[R] T)
) with the first projection is
the morphism Spec (S ⊗[R] T) ⟶ Spec S
obtained by applying Spec.map
to the ring morphism
s ↦ s ⊗ₜ[R] 1
.
The composition of the inverse of the isomorphism pullbackSepcIso R S T
(from the pullback of
Spec S ⟶ Spec R
and Spec T ⟶ Spec R
to Spec (S ⊗[R] T)
) with the second projection is
the morphism Spec (S ⊗[R] T) ⟶ Spec T
obtained by applying Spec.map
to the ring morphism
t ↦ 1 ⊗ₜ[R] t
.
The composition of the isomorphism pullbackSepcIso R S T
(from the pullback of
Spec S ⟶ Spec R
and Spec T ⟶ Spec R
to Spec (S ⊗[R] T)
) with the morphism
Spec (S ⊗[R] T) ⟶ Spec S
obtained by applying Spec.map
to the ring morphism s ↦ s ⊗ₜ[R] 1
is the first projection.
The composition of the isomorphism pullbackSepcIso R S T
(from the pullback of
Spec S ⟶ Spec R
and Spec T ⟶ Spec R
to Spec (S ⊗[R] T)
) with the morphism
Spec (S ⊗[R] T) ⟶ Spec T
obtained by applying Spec.map
to the ring morphism t ↦ 1 ⊗ₜ[R] t
is the second projection.