Partially defined linear maps #
A LinearPMap R E F or E →ₗ.[R] F is a linear map from a submodule of E to F.
We define a SemilatticeInf with OrderBot instance on this, and define three operations:
mkSpanSingletondefines a partial linear map defined on the span of a singleton.suptakes two partial linear mapsf,gthat agree on the intersection of their domains, and returns the unique partial linear map onf.domain ⊔ g.domainthat extends bothfandg.sSuptakes aDirectedOn (· ≤ ·)set of partial linear maps, and returns the unique partial linear map on thesSupof their domains that extends all these maps.
Moreover, we define
LinearPMap.graphis the graph of the partial linear map viewed as a submodule ofE × F.
Partially defined maps are currently used in Mathlib to prove Hahn-Banach theorem
and its variations. Namely, LinearPMap.sSup implies that every chain of LinearPMaps
is bounded above.
They are also the basis for the theory of unbounded operators.
A LinearPMap R E F or E →ₗ.[R] F is a linear map from a submodule of E to F.
- domain : Submodule R E
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The unique LinearPMap on R ∙ x that sends x to y. This version works for modules
over rings, and requires a proof of ∀ c, c • x = 0 → c • y = 0.
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The unique LinearPMap on span R {x} that sends a non-zero vector x to y.
This version works for modules over division rings.
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Projection to the first coordinate as a LinearPMap
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Projection to the second coordinate as a LinearPMap
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Given two partial linear maps f, g, the set of points x such that
both f and g are defined at x and f x = g x form a submodule.
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Given two partial linear maps that agree on the intersection of their domains,
f.sup g h is the unique partial linear map on f.domain ⊔ g.domain that agrees
with f and g.
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Hypothesis for LinearPMap.sup holds, if f.domain is disjoint with g.domain.
Algebraic operations #
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Extend a LinearPMap to f.domain ⊔ K ∙ x.
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Restrict a linear map to a submodule, reinterpreting the result as a LinearPMap.
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Compose a linear map with a LinearPMap
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Restrict codomain of a LinearPMap
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Compose two LinearPMaps
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f.coprod g is the partially defined linear map defined on f.domain × g.domain,
and sending p to f p.1 + g p.2.
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Graph #
The graph of a LinearPMap viewed as a submodule on E × F.
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The graph of z • f as a pushforward.
The graph of -f as a pushforward.
Auxiliary definition to unfold the existential quantifier.
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Define a LinearMap from its graph.
Helper definition for LinearPMap.
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Define a LinearPMap from its graph.
In the case that the submodule is not a graph of a LinearPMap then the underlying linear map
is just the zero map.
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The inverse of a LinearPMap.
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The graph of the inverse generates a LinearPMap.