Affine equivalences #
In this file we define AffineEquiv k P₁ P₂ (notation: P₁ ≃ᵃ[k] P₂) to be the type of affine
equivalences between P₁ and P₂, i.e., equivalences such that both forward and inverse maps are
affine maps.
We define the following equivalences:
AffineEquiv.refl k P: the identity map as anAffineEquiv;e.symm: the inverse map of anAffineEquivas anAffineEquiv;e.trans e': composition of twoAffineEquivs; note that the order followsmathlib'sCategoryTheoryconvention (applye, thene'), not the convention used in function composition and compositions of bundled morphisms.
We equip AffineEquiv k P P with a Group structure with multiplication corresponding to
composition in AffineEquiv.group.
Tags #
affine space, affine equivalence
An affine equivalence, denoted P₁ ≃ᵃ[k] P₂, is an equivalence between affine spaces
such that both forward and inverse maps are affine.
We define it using an Equiv for the map and a LinearEquiv for the linear part in order
to allow affine equivalences with good definitional equalities.
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An affine equivalence, denoted P₁ ≃ᵃ[k] P₂, is an equivalence between affine spaces
such that both forward and inverse maps are affine.
We define it using an Equiv for the map and a LinearEquiv for the linear part in order
to allow affine equivalences with good definitional equalities.
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Reinterpret an AffineEquiv as an AffineMap.
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Construct an affine equivalence by verifying the relation between the map and its linear part at
one base point. Namely, this function takes a map e : P₁ → P₂, a linear equivalence
e' : V₁ ≃ₗ[k] V₂, and a point p such that for any other point p' we have
e p' = e' (p' -ᵥ p) +ᵥ e p.
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Inverse of an affine equivalence as an affine equivalence.
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See Note [custom simps projection]
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See Note [custom simps projection]
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Bijective affine maps are affine isomorphisms.
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Identity map as an AffineEquiv.
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Composition of two AffineEquivalences, applied left to right.
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AffineEquiv.linear on automorphisms is a MonoidHom.
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The group of AffineEquivs are equivalent to the group of units of AffineMap.
This is the affine version of LinearMap.GeneralLinearGroup.generalLinearEquiv.
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Product of two affine equivalences. The map comes from Equiv.prodCongr
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Product of affine spaces is commutative up to affine isomorphism.
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Product of affine spaces is associative up to affine isomorphism.
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The map v ↦ v +ᵥ b as an affine equivalence between a module V and an affine space P with
tangent space V.
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p' ↦ p -ᵥ p' as an equivalence.
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The map p ↦ v +ᵥ p as an affine automorphism of an affine space.
Note that there is no need for an AffineMap.constVAdd as it is always an equivalence.
This is roughly to DistribMulAction.toLinearEquiv as +ᵥ is to •.
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A more bundled version of AffineEquiv.constVAdd.
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Fixing a point in affine space, homothety about this point gives a group homomorphism from (the centre of) the units of the scalars into the group of affine equivalences.
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Point reflection in x as a permutation.
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x is the only fixed point of pointReflection x. This lemma requires
x + x = y + y ↔ x = y. There is no typeclass to use here, so we add it as an explicit argument.
Interpret a linear equivalence between modules as an affine equivalence.
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Construct an affine equivalence from a linear equivalence and two base points.
Given a linear equivalence A : V ≃ₗ[k] V and base points p₀ p₁ : P, this constructs
the affine equivalence T x = A (x -ᵥ p₀) +ᵥ p₁. This is the standard way to convert
a linear automorphism into an affine automorphism with specified base point mapping.
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Affine isomorphisms between the domains and codomains of two spaces of affine maps give a bijection between the two function spaces.
See AffineEquiv.arrowCongr and AffineEquiv.arrowCongrₗ for the affine and linear versions of
this bijection.
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An affine isomorphism between the domains and a linear isomorphism between the codomains of two spaces of affine maps give a linear isomorphism between the two function spaces.
See also AffineEquiv.arrowCongrEquiv and AffineEquiv.arrowCongr.
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Affine isomorphisms between the domains and codomains of two spaces of affine maps give an affine isomorphism between the two function spaces.
See also AffineEquiv.arrowCongrEquiv and AffineEquiv.arrowCongrₗ.
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An affine isomorphism between the domains of affine spaces induces a linear isomorphism over another ring between the two function spaces.
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An affine isomorphism between the domains of affine spaces induces an affine isomorphism over another ring between the two function spaces.