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⁆ N
for a morphism of Lie algebra modulesM
,N
over a Lie algebraL
,M ≃ₗ⁅R,L⁆ N
for an equivalence of Lie algebra modulesM
,N
over 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
- neg : L → L
- sub : L → 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.
Equations
Instances For
Equations
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
.
Equations
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.
Instances For
A morphism of Lie algebras (denoted as L₁ →ₗ⁅R⁆ L₂
)
is a linear map respecting the bracket operations.
Equations
Instances For
Equations
Equations
The constant 0 map is a Lie algebra morphism.
Equations
The identity map is a Lie algebra morphism.
Equations
Equations
The composition of morphisms is a morphism.
Equations
Instances For
The inverse of a bijective morphism is a morphism.
Equations
Instances For
A Lie ring module may be pulled back along a morphism of Lie algebras.
See note [reducible non-instances].
Equations
Instances For
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
- left_inv : Function.LeftInverse self.invFun (↑self.toLieHom).toFun
The inverse function of an equivalence of Lie algebras is a left inverse of the underlying function.
- right_inv : Function.RightInverse self.invFun (↑self.toLieHom).toFun
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.
Equations
Instances For
Consider an equivalence of Lie algebras as a linear equivalence.
Equations
Instances For
Equations
Equations
Equations
Equations
Equations
Lie algebra equivalences are reflexive.
Equations
Instances For
Lie algebra equivalences are symmetric.
Equations
Instances For
Lie algebra equivalences are transitive.
Equations
Instances For
A bijective morphism of Lie algebras yields an equivalence of Lie algebras.
Equations
Instances For
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.
Instances For
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.
Equations
Instances For
Equations
Equations
The identity map is a morphism of Lie modules.
Equations
Instances For
The constant 0 map is a Lie module morphism.
Equations
The identity map is a Lie module morphism.
Equations
Equations
The composition of Lie module morphisms is a morphism.
Equations
Instances For
The inverse of a bijective morphism of Lie modules is a morphism of Lie modules.
Equations
Instances For
Equations
Equations
Equations
Equations
Equations
Equations
Equations
Equations
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.
Equations
Instances For
View an equivalence of Lie modules as a linear equivalence.
Equations
Instances For
View an equivalence of Lie modules as a type level equivalence.
Equations
Instances For
Equations
Equations
Equations
Equations
Equations
Equations
Lie module equivalences are reflexive.
Equations
Instances For
Lie module equivalences are symmetric.
Equations
Instances For
Lie module equivalences are transitive.