# Homomorphisms of Lie Algebras¶

AUTHORS:

• Travis Scrimshaw (07-15-2013): Initial implementation
• Eero Hakavuori (08-09-2018): Morphisms defined by a generating subset
class sage.algebras.lie_algebras.morphism.LieAlgebraHomomorphism_im_gens(parent, im_gens, check=True)

A homomorphism of Lie algebras.

Let $$\mathfrak{g}$$ and $$\mathfrak{g}^{\prime}$$ be Lie algebras. A linear map $$f \colon \mathfrak{g} \to \mathfrak{g}^{\prime}$$ is a homomorphism (of Lie algebras) if $$f([x, y]) = [f(x), f(y)]$$ for all $$x, y \in \mathfrak{g}$$. Thus homomorphisms are completely determined by the image of the generators of $$\mathfrak{g}$$.

EXAMPLES:

sage: L = LieAlgebra(QQ, 'x,y,z')
sage: Lyn = L.Lyndon()
sage: H = L.Hall()
doctest:warning...:
FutureWarning: The Hall basis has not been fully proven correct, but currently no bugs are known
See http://trac.sagemath.org/16823 for details.
sage: phi = Lyn.coerce_map_from(H); phi
Lie algebra morphism:
From: Free Lie algebra generated by (x, y, z) over Rational Field in the Hall basis
To:   Free Lie algebra generated by (x, y, z) over Rational Field in the Lyndon basis
Defn: x |--> x
y |--> y
z |--> z

im_gens()

Return the images of the generators of the domain.

OUTPUT:

• list – a copy of the list of gens (it is safe to change this)

EXAMPLES:

sage: L = LieAlgebra(QQ, 'x,y,z')
sage: Lyn = L.Lyndon()
sage: H = L.Hall()
sage: f = Lyn.coerce_map_from(H)
sage: f.im_gens()
[x, y, z]

class sage.algebras.lie_algebras.morphism.LieAlgebraHomset(X, Y, category=None, base=None, check=True)

Homset between two Lie algebras.

Todo

This is a very minimal implementation which does not have coercions of the morphisms.

zero()

Return the zero morphism.

EXAMPLES:

sage: L = LieAlgebra(QQ, 'x,y,z')
sage: Lyn = L.Lyndon()
sage: H = L.Hall()
sage: HS = Hom(Lyn, H)
sage: HS.zero()
Generic morphism:
From: Free Lie algebra generated by (x, y, z) over Rational Field in the Lyndon basis
To:   Free Lie algebra generated by (x, y, z) over Rational Field in the Hall basis

class sage.algebras.lie_algebras.morphism.LieAlgebraMorphism_from_generators(on_generators, domain=None, codomain=None, check=True)

A morphism between two Lie algebras defined by images of a generating set as a Lie algebra.

This is the Lie algebra morphism $$\phi \colon L \to K$$ defined on the chosen basis of $$L$$ to that of $$K$$ be using the image of some generating set (as a Lie algebra) of $$L$$.

INPUT:

• on_generators – dictionary {X: Y} of the images $$Y$$ in codomain of elements $$X$$ of domain
• codomain – a Lie algebra (optional); this is inferred from the values of on_generators if not given
• check – (default: True) boolean; if False the values on the Lie brackets implied by on_generators will not be checked for contradictory values

EXAMPLES:

A reflection of one horizontal vector in the Heisenberg algebra:

sage: L.<X,Y,Z> = LieAlgebra(QQ, {('X','Y'): {'Z':1}})
sage: phi = L.morphism({X:-X, Y:Y}); phi
Lie algebra endomorphism of Lie algebra on 3 generators (X, Y, Z) over Rational Field
Defn: X |--> -X
Y |--> Y
Z |--> -Z


There is no Lie algebra morphism that reflects one horizontal vector, but not the vertical one:

sage: L.morphism({X:-X, Y:Y, Z:Z})
Traceback (most recent call last):
...
ValueError: this does not define a Lie algebra morphism;
contradictory values for brackets of length 2


Checking for mistakes can be disabled, which can produce invalid results:

sage: phi = L.morphism({X:-X, Y:Y, Z:Z}, check=False); phi
Lie algebra endomorphism of Lie algebra on 3 generators (X, Y, Z) over Rational Field
Defn: X |--> -X
Y |--> Y
Z |--> Z
sage: L[phi(X), phi(Y)] == phi(L[X,Y])
False


The set of keys must generate the Lie algebra:

sage: L.morphism({X: X})
Traceback (most recent call last):
...
ValueError: [X] is not a generating set of Lie algebra on 3 generators
(X, Y, Z) over Rational Field


Over non-fields, generating subsets are more restricted:

sage: L.<X,Y,Z> = LieAlgebra(ZZ, {('X','Y'): {'Z':2}})
sage: L.morphism({X: X, Y: Y})
Traceback (most recent call last):
...
ValueError: [X, Y] is not a generating set of Lie algebra on 3
generators (X, Y, Z) over Integer Ring


The generators do not have to correspond to the defined generating set of the domain:

sage: L.<X,Y,Z,W> = LieAlgebra(QQ, {('X','Y'): {'Z':1}, ('X','Z'): {'W':1}})
sage: K.<A,B,C> = LieAlgebra(QQ, {('A','B'): {'C':2}})
sage: phi = L.morphism({X+2*Y: A, X-Y: B}); phi
Lie algebra morphism:
From: Lie algebra on 4 generators (X, Y, Z, W) over Rational Field
To:   Lie algebra on 3 generators (A, B, C) over Rational Field
Defn: X |--> 1/3*A + 2/3*B
Y |--> 1/3*A - 1/3*B
Z |--> -2/3*C
W |--> 0
sage: phi(X+2*Y)
A
sage: phi(X)
1/3*A + 2/3*B
sage: phi(W)
0
sage: phi(Z)
-2/3*C
sage: all(K[phi(p), phi(q)] == phi(L[p,q])
....:     for p in L.basis() for q in L.basis())
True


A quotient type Lie algebra morphism:

sage: K.<A,B> = LieAlgebra(SR, abelian=True)
sage: L.morphism({X: A, Y: B})
Lie algebra morphism:
From: Lie algebra on 4 generators (X, Y, Z, W) over Rational Field
To:   Abelian Lie algebra on 2 generators (A, B) over Symbolic Ring
Defn: X |--> A
Y |--> B
Z |--> 0
W |--> 0