The lattice of subobjects #
We provide the SemilatticeInf with OrderTop (Subobject X) instance when [HasPullback C],
and the SemilatticeSup (Subobject X) instance when [HasImages C] [HasBinaryCoproducts C].
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The morphism to the top object in MonoOver X.
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The pullback of the top object in MonoOver Y
is (isomorphic to) the top object in MonoOver X.
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There is a morphism from ⊤ : MonoOver A to the pullback of a monomorphism along itself;
as the category is thin this is an isomorphism.
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The pullback of a monomorphism along itself is isomorphic to the top object.
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The (unique) morphism from ⊥ : MonoOver X to any other f : MonoOver X.
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map f sends ⊥ : MonoOver X to ⊥ : MonoOver Y.
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The object underlying ⊥ : Subobject B is (up to isomorphism) the zero object.
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When [HasPullbacks C], MonoOver A has "intersections", functorial in both arguments.
As MonoOver A is only a preorder, this doesn't satisfy the axioms of SemilatticeInf,
but we reuse all the names from SemilatticeInf because they will be used to construct
SemilatticeInf (subobject A) shortly.
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A morphism from the "infimum" of two objects in MonoOver A to the first object.
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A morphism from the "infimum" of two objects in MonoOver A to the second object.
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A morphism version of the le_inf axiom.
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When [HasImages C] [HasBinaryCoproducts C], MonoOver A has a sup construction,
which is functorial in both arguments,
and which on Subobject A will induce a SemilatticeSup.
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A morphism version of le_sup_left.
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A morphism version of le_sup_right.
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A morphism version of sup_le.
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The object underlying ⊥ : Subobject B is (up to isomorphism) the initial object.
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The object underlying ⊥ : Subobject B is (up to isomorphism) the zero object.
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Sending X : C to Subobject X is a contravariant functor Cᵒᵖ ⥤ Type.
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The functorial infimum on MonoOver A descends to an infimum on Subobject A.
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⊓ commutes with pullback.
⊓ commutes with map.
The functorial supremum on MonoOver A descends to a supremum on Subobject A.
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The "wide cospan" diagram, with a small indexing type, constructed from a set of subobjects.
(This is just the diagram of all the subobjects pasted together, but using WellPowered C
to make the diagram small.)
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Auxiliary construction of a cone for le_inf.
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The limit of wideCospan s. (This will be the supremum of the set of subobjects.)
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The inclusion map from widePullback s to A
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When [WellPowered C] and [HasWidePullbacks C], Subobject A has arbitrary infimums.
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The universal morphism out of the coproduct of a set of subobjects,
after using [WellPowered C] to reindex by a small type.
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When [WellPowered C] [HasImages C] [HasCoproducts C],
Subobject A has arbitrary supremums.
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A nonzero object has nontrivial subobject lattice.
The subobject lattice of a subobject Y is order isomorphic to the interval Set.Iic Y.