Lie algebras #
This file defines Lie rings and Lie algebras over a commutative ring together with their modules, morphisms and equivalences, as well as various lemmas to make these definitions usable.
Main definitions #
Notation #
Working over a fixed commutative ring R, we introduce the notations:
L →ₗ⁅R⁆ L'for a morphism of Lie algebras,L ≃ₗ⁅R⁆ L'for an equivalence of Lie algebras,M →ₗ⁅R,L⁆ Nfor a morphism of Lie algebra modulesM,Nover a Lie algebraL,M ≃ₗ⁅R,L⁆ Nfor an equivalence of Lie algebra modulesM,Nover a Lie algebraL.
Implementation notes #
Lie algebras are defined as modules with a compatible Lie ring structure and thus, like modules, are partially unbundled.
References #
Tags #
lie bracket, jacobi identity, lie ring, lie algebra, lie module
A Lie ring is an additive group with compatible product, known as the bracket, satisfying the Jacobi identity.
- add : L → L → L
- zero : L
- nsmul : ℕ → L → L
- neg : L → L
- sub : L → L → L
- zsmul : ℤ → L → L
- bracket : L → L → L
A Lie ring bracket is additive in its first component.
A Lie ring bracket is additive in its second component.
A Lie ring bracket vanishes on the diagonal in L × L.
A Lie ring bracket satisfies a Leibniz / Jacobi identity.
Instances
A Lie algebra is a module with compatible product, known as the bracket, satisfying the Jacobi identity. Forgetting the scalar multiplication, every Lie algebra is a Lie ring.
- smul : R → L → L
A Lie algebra bracket is compatible with scalar multiplication in its second argument.
The compatibility in the first argument is not a class property, but follows since every Lie algebra has a natural Lie module action on itself, see
LieModule.
Instances
A Lie ring module is an additive group, together with an additive action of a
Lie ring on this group, such that the Lie bracket acts as the commutator of endomorphisms.
(For representations of Lie algebras see LieModule.)
- bracket : L → M → M
A Lie ring module bracket is additive in its first component.
A Lie ring module bracket is additive in its second component.
A Lie ring module bracket satisfies a Leibniz / Jacobi identity.
Instances
A Lie module is a module over a commutative ring, together with a linear action of a Lie algebra on this module, such that the Lie bracket acts as the commutator of endomorphisms.
A Lie module bracket is compatible with scalar multiplication in its first argument.
A Lie module bracket is compatible with scalar multiplication in its second argument.
Instances
A tower of Lie bracket actions encapsulates the Leibniz rule for Lie bracket actions.
More precisely, it does so in a relative setting:
Let L₁ and L₂ be two types with Lie bracket actions on a type M endowed with an addition,
and additionally assume a Lie bracket action of L₁ on L₂.
Then the Leibniz rule asserts for all x : L₁, y : L₂, and m : M that
⁅x, ⁅y, m⁆⁆ = ⁅⁅x, y⁆, m⁆ + ⁅y, ⁅x, m⁆⁆ holds.
Common examples include the case where L₁ is a Lie subalgebra of L₂
and the case where L₂ is a Lie ideal of L₁.
Instances
Every Lie algebra is a module over itself.
The Lie bracket as a biadditive map.
Usually one will have coefficients and LieModule.toEnd will be more useful.
Instances For
We could avoid defining this by instead defining a LieRingModule L R instance with a zero
bracket and relying on LinearMap.instLieRingModule. We do not do this because in the case that
L = R we would have a non-defeq diamond via Ring.instBracket.
It is sometimes useful to regard a LieRing as a NonUnitalNonAssocRing.
Instances For
A morphism of Lie algebras (denoted as L₁ →ₗ⁅R⁆ L₂)
is a linear map respecting the bracket operations.
- toFun : L → L'
A morphism of Lie algebras is compatible with brackets.
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A morphism of Lie algebras (denoted as L₁ →ₗ⁅R⁆ L₂)
is a linear map respecting the bracket operations.
Instances For
Alias of map_smul.
Alias of map_add.
Alias of map_sub.
Alias of map_neg.
Alias of map_zero.
The identity map is a morphism of Lie algebras.
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The constant 0 map is a Lie algebra morphism.
The identity map is a Lie algebra morphism.
The composition of morphisms is a morphism.
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The inverse of a bijective morphism is a morphism.
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A Lie ring module may be pulled back along a morphism of Lie algebras.
See note [reducible non-instances].
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A Lie module may be pulled back along a morphism of Lie algebras.
An equivalence of Lie algebras (denoted as L₁ ≃ₗ⁅R⁆ L₂) is a morphism
which is also a linear equivalence.
We could instead define an equivalence to be a morphism which is also a (plain) equivalence.
However, it is more convenient to define via linear equivalence to get .toLinearEquiv for free.
- toFun : L → L'
- invFun : L' → L
The inverse function of an equivalence of Lie algebras
The inverse function of an equivalence of Lie algebras is a left inverse of the underlying function.
The inverse function of an equivalence of Lie algebras is a right inverse of the underlying function.
Instances For
An equivalence of Lie algebras (denoted as L₁ ≃ₗ⁅R⁆ L₂) is a morphism
which is also a linear equivalence.
We could instead define an equivalence to be a morphism which is also a (plain) equivalence.
However, it is more convenient to define via linear equivalence to get .toLinearEquiv for free.
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Consider an equivalence of Lie algebras as a linear equivalence.
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Lie algebra equivalences are reflexive.
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Lie algebra equivalences are symmetric.
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Lie algebra equivalences are transitive.
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A bijective morphism of Lie algebras yields an equivalence of Lie algebras.
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A morphism of Lie algebra modules (denoted as M →ₗ⁅R,L⁆ N) is a linear map
which commutes with the action of the Lie algebra.
- toFun : M → N
A module of Lie algebra modules is compatible with the action of the Lie algebra on the modules.
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A morphism of Lie algebra modules (denoted as M →ₗ⁅R,L⁆ N) is a linear map
which commutes with the action of the Lie algebra.
Instances For
Alias of map_smul.
Alias of map_add.
Alias of map_sub.
Alias of map_neg.
Alias of map_zero.
The identity map is a morphism of Lie modules.
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The constant 0 map is a Lie module morphism.
The identity map is a Lie module morphism.
The composition of Lie module morphisms is a morphism.
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The inverse of a bijective morphism of Lie modules is a morphism of Lie modules.
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An equivalence of Lie algebra modules (denoted as M ≃ₗ⁅R,L⁆ N) is a linear equivalence
which is also a morphism of Lie algebra modules.
- toFun : M → N
- map_add' (x y : M) : (↑self.toLieModuleHom).toFun (x + y) = (↑self.toLieModuleHom).toFun x + (↑self.toLieModuleHom).toFun y
- map_smul' (m : R) (x : M) : (↑self.toLieModuleHom).toFun (m • x) = (RingHom.id R) m • (↑self.toLieModuleHom).toFun x
- invFun : N → M
The inverse function of an equivalence of Lie modules
- left_inv : Function.LeftInverse self.invFun (↑self.toLieModuleHom).toFun
The inverse function of an equivalence of Lie modules is a left inverse of the underlying function.
- right_inv : Function.RightInverse self.invFun (↑self.toLieModuleHom).toFun
The inverse function of an equivalence of Lie modules is a right inverse of the underlying function.
Instances For
An equivalence of Lie algebra modules (denoted as M ≃ₗ⁅R,L⁆ N) is a linear equivalence
which is also a morphism of Lie algebra modules.
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View an equivalence of Lie modules as a linear equivalence.
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View an equivalence of Lie modules as a type level equivalence.
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Lie module equivalences are reflexive.
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Lie module equivalences are symmetric.
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Lie module equivalences are transitive.