# Examples of Lie Algebras¶

There are the following examples of Lie algebras:

• A rather comprehensive family of 3-dimensional Lie algebras
• The Lie algebra of affine transformations of the line
• All abelian Lie algebras on free modules
• The Lie algebra of upper triangular matrices
• The Lie algebra of strictly upper triangular matrices

AUTHORS:

• Travis Scrimshaw (07-15-2013): Initial implementation
sage.algebras.lie_algebras.examples.Heisenberg(R, n, representation='structure')

Return the rank n Heisenberg algebra in the given representation.

INPUT:

• R – the base ring
• n – the rank (a nonnegative integer or infinity)
• representation – (default: “structure”) can be one of the following:
• "structure" – using structure coefficients
• "matrix" – using matrices

EXAMPLES:

sage: lie_algebras.Heisenberg(QQ, 3)
Heisenberg algebra of rank 3 over Rational Field

sage.algebras.lie_algebras.examples.abelian(R, names=None, index_set=None)

Return the abelian Lie algebra generated by names.

EXAMPLES:

sage: lie_algebras.abelian(QQ, 'x, y, z')
Abelian Lie algebra on 3 generators (x, y, z) over Rational Field

sage.algebras.lie_algebras.examples.affine_transformations_line(R, names=['X', 'Y'], representation='bracket')

The Lie algebra of affine transformations of the line.

EXAMPLES:

sage: L = lie_algebras.affine_transformations_line(QQ)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Y}
sage: X, Y = L.lie_algebra_generators()
sage: L[X, Y] == Y
True
sage: TestSuite(L).run()
sage: L = lie_algebras.affine_transformations_line(QQ, representation="matrix")
sage: X, Y = L.lie_algebra_generators()
sage: L[X, Y] == Y
True
sage: TestSuite(L).run()

sage.algebras.lie_algebras.examples.cross_product(R, names=['X', 'Y', 'Z'])

The Lie algebra of $$\RR^3$$ defined by the usual cross product $$\times$$.

EXAMPLES:

sage: L = lie_algebras.cross_product(QQ)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Z, ('X', 'Z'): -Y, ('Y', 'Z'): X}
sage: TestSuite(L).run()

sage.algebras.lie_algebras.examples.pwitt(R, p)

Return the $$p$$-Witt Lie algebra over $$R$$.

INPUT:

• R – the base ring
• p – a positive integer that is $$0$$ in R

EXAMPLES:

sage: lie_algebras.pwitt(GF(5), 5)
The 5-Witt Lie algebra over Finite Field of size 5

sage.algebras.lie_algebras.examples.regular_vector_fields(R)

Return the Lie algebra of regular vector fields on $$\CC^{\times}$$.

This is also known as the Witt (Lie) algebra.

EXAMPLES:

sage: lie_algebras.regular_vector_fields(QQ)
The Lie algebra of regular vector fields over Rational Field

sage.algebras.lie_algebras.examples.sl(R, n, representation='bracket')

The Lie algebra $$\mathfrak{sl}_n$$.

The Lie algebra $$\mathfrak{sl}_n$$ is the type $$A_{n-1}$$ Lie algebra and is finite dimensional. As a matrix Lie algebra, it is given by the set of all $$n \times n$$ matrices with trace 0.

INPUT:

• R – the base ring
• n – the size of the matrix
• representation – (default: 'bracket') can be one of the following:
• 'bracket' - use brackets and the Chevalley basis
• 'matrix' - use matrices

EXAMPLES:

We first construct $$\mathfrak{sl}_2$$ using the Chevalley basis:

sage: sl2 = lie_algebras.sl(QQ, 2); sl2
Lie algebra of ['A', 1] in the Chevalley basis
sage: E,F,H = sl2.gens()
sage: E.bracket(F) == H
True
sage: H.bracket(E) == 2*E
True
sage: H.bracket(F) == -2*F
True


We now construct $$\mathfrak{sl}_2$$ as a matrix Lie algebra:

sage: sl2 = lie_algebras.sl(QQ, 2, representation='matrix')
sage: E,F,H = sl2.gens()
sage: E.bracket(F) == H
True
sage: H.bracket(E) == 2*E
True
sage: H.bracket(F) == -2*F
True

sage.algebras.lie_algebras.examples.so(R, n, representation='bracket')

The Lie algebra $$\mathfrak{so}_n$$.

The Lie algebra $$\mathfrak{so}_n$$ is the type $$B_k$$ Lie algebra if $$n = 2k - 1$$ or the type $$D_k$$ Lie algebra if $$n = 2k$$, and in either case is finite dimensional. As a matrix Lie algebra, it is given by the set of all real anti-symmetric $$n \times n$$ matrices.

INPUT:

• R – the base ring
• n – the size of the matrix
• representation – (default: 'bracket') can be one of the following:
• 'bracket' - use brackets and the Chevalley basis
• 'matrix' - use matrices

EXAMPLES:

We first construct $$\mathfrak{so}_5$$ using the Chevalley basis:

sage: so5 = lie_algebras.so(QQ, 5); so5
Lie algebra of ['B', 2] in the Chevalley basis
sage: E1,E2, F1,F2, H1,H2 = so5.gens()
sage: so5([E1, [E1, E2]])
0
sage: X = so5([E2, [E2, E1]]); X
-2*E[alpha[1] + 2*alpha[2]]
sage: H1.bracket(X)
0
sage: H2.bracket(X)
-4*E[alpha[1] + 2*alpha[2]]
sage: so5([H1, [E1, E2]])
-E[alpha[1] + alpha[2]]
sage: so5([H2, [E1, E2]])
0


We do the same construction of $$\mathfrak{so}_4$$ using the Chevalley basis:

sage: so4 = lie_algebras.so(QQ, 4); so4
Lie algebra of ['D', 2] in the Chevalley basis
sage: E1,E2, F1,F2, H1,H2 = so4.gens()
sage: H1.bracket(E1)
2*E[alpha[1]]
sage: H2.bracket(E1) == so4.zero()
True
sage: E1.bracket(E2) == so4.zero()
True


We now construct $$\mathfrak{so}_4$$ as a matrix Lie algebra:

sage: sl2 = lie_algebras.sl(QQ, 2, representation='matrix')
sage: E1,E2, F1,F2, H1,H2 = so4.gens()
sage: H2.bracket(E1) == so4.zero()
True
sage: E1.bracket(E2) == so4.zero()
True

sage.algebras.lie_algebras.examples.sp(R, n, representation='bracket')

The Lie algebra $$\mathfrak{sp}_n$$.

The Lie algebra $$\mathfrak{sp}_n$$ where $$n = 2k$$ is the type $$C_k$$ Lie algebra and is finite dimensional. As a matrix Lie algebra, it is given by the set of all matrices $$X$$ that satisfy the equation:

$X^T M - M X = 0$

where

$\begin{split}M = \begin{pmatrix} 0 & I_k \\ -I_k & 0 \end{pmatrix}.\end{split}$

This is the Lie algebra of type $$C_k$$.

INPUT:

• R – the base ring
• n – the size of the matrix
• representation – (default: 'bracket') can be one of the following:
• 'bracket' - use brackets and the Chevalley basis
• 'matrix' - use matrices

EXAMPLES:

We first construct $$\mathfrak{sp}_4$$ using the Chevalley basis:

sage: sp4 = lie_algebras.sp(QQ, 4); sp4
Lie algebra of ['C', 2] in the Chevalley basis
sage: E1,E2, F1,F2, H1,H2 = sp4.gens()
sage: sp4([E2, [E2, E1]])
0
sage: X = sp4([E1, [E1, E2]]); X
2*E[2*alpha[1] + alpha[2]]
sage: H1.bracket(X)
4*E[2*alpha[1] + alpha[2]]
sage: H2.bracket(X)
0
sage: sp4([H1, [E1, E2]])
0
sage: sp4([H2, [E1, E2]])
-E[alpha[1] + alpha[2]]


We now construct $$\mathfrak{sp}_4$$ as a matrix Lie algebra:

sage: sp4 = lie_algebras.sp(QQ, 4, representation='matrix'); sp4
Symplectic Lie algebra of rank 4 over Rational Field
sage: E1,E2, F1,F2, H1,H2 = sp4.gens()
sage: H1.bracket(E1)
[ 0  2  0  0]
[ 0  0  0  0]
[ 0  0  0  0]
[ 0  0 -2  0]
sage: sp4([E1, [E1, E2]])
[0 0 2 0]
[0 0 0 0]
[0 0 0 0]
[0 0 0 0]

sage.algebras.lie_algebras.examples.strictly_upper_triangular_matrices(R, n)

Return the Lie algebra $$\mathfrak{n}_k$$ of strictly $$k \times k$$ upper triangular matrices.

Todo

This implementation does not know it is finite-dimensional and does not know its basis.

EXAMPLES:

sage: L = lie_algebras.strictly_upper_triangular_matrices(QQ, 4); L
Lie algebra of 4-dimensional strictly upper triangular matrices over Rational Field
sage: TestSuite(L).run()
sage: n0, n1, n2 = L.lie_algebra_generators()
sage: L[n2, n1]
[ 0  0  0  0]
[ 0  0  0 -1]
[ 0  0  0  0]
[ 0  0  0  0]

sage.algebras.lie_algebras.examples.three_dimensional(R, a, b, c, d, names=['X', 'Y', 'Z'])

The 3-dimensional Lie algebra over a given commutative ring $$R$$ with basis $$\{X, Y, Z\}$$ subject to the relations:

$[X, Y] = aZ + dY, \quad [Y, Z] = bX, \quad [Z, X] = cY + dZ$

where $$a,b,c,d \in R$$.

This is always a well-defined 3-dimensional Lie algebra, as can be easily proven by computation.

EXAMPLES:

sage: L = lie_algebras.three_dimensional(QQ, 4, 1, -1, 2)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): 2*Y + 4*Z, ('X', 'Z'): Y - 2*Z, ('Y', 'Z'): X}
sage: TestSuite(L).run()
sage: L = lie_algebras.three_dimensional(QQ, 1, 0, 0, 0)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Z}
sage: L = lie_algebras.three_dimensional(QQ, 0, 0, -1, -1)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): -Y, ('X', 'Z'): Y + Z}
sage: L = lie_algebras.three_dimensional(QQ, 0, 1, 0, 0)
sage: L.structure_coefficients()
Finite family {('Y', 'Z'): X}
sage: lie_algebras.three_dimensional(QQ, 0, 0, 0, 0)
Abelian Lie algebra on 3 generators (X, Y, Z) over Rational Field
sage: Q.<a,b,c,d> = PolynomialRing(QQ)
sage: L = lie_algebras.three_dimensional(Q, a, b, c, d)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): d*Y + a*Z, ('X', 'Z'): (-c)*Y + (-d)*Z, ('Y', 'Z'): b*X}
sage: TestSuite(L).run()

sage.algebras.lie_algebras.examples.three_dimensional_by_rank(R, n, a=None, names=['X', 'Y', 'Z'])

Return a 3-dimensional Lie algebra of rank n, where $$0 \leq n \leq 3$$.

Here, the rank of a Lie algebra $$L$$ is defined as the dimension of its derived subalgebra $$[L, L]$$. (We are assuming that $$R$$ is a field of characteristic $$0$$; otherwise the Lie algebras constructed by this function are still well-defined but no longer might have the correct ranks.) This is not to be confused with the other standard definition of a rank (namely, as the dimension of a Cartan subalgebra, when $$L$$ is semisimple).

INPUT:

• R – the base ring
• n – the rank
• a – the deformation parameter (used for $$n = 2$$); this should be a nonzero element of $$R$$ in order for the resulting Lie algebra to actually have the right rank(?)
• names – (optional) the generator names

EXAMPLES:

sage: lie_algebras.three_dimensional_by_rank(QQ, 0)
Abelian Lie algebra on 3 generators (X, Y, Z) over Rational Field
sage: L = lie_algebras.three_dimensional_by_rank(QQ, 1)
sage: L.structure_coefficients()
Finite family {('Y', 'Z'): X}
sage: L = lie_algebras.three_dimensional_by_rank(QQ, 2, 4)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Y, ('X', 'Z'): Y + Z}
sage: L = lie_algebras.three_dimensional_by_rank(QQ, 2, 0)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Y}
sage: lie_algebras.three_dimensional_by_rank(QQ, 3)
sl2 over Rational Field

sage.algebras.lie_algebras.examples.upper_triangular_matrices(R, n)

Return the Lie algebra $$\mathfrak{b}_k$$ of $$k \times k$$ upper triangular matrices.

Todo

This implementation does not know it is finite-dimensional and does not know its basis.

EXAMPLES:

sage: L = lie_algebras.upper_triangular_matrices(QQ, 4); L
Lie algebra of 4-dimensional upper triangular matrices over Rational Field
sage: TestSuite(L).run()
sage: n0, n1, n2, t0, t1, t2, t3 = L.lie_algebra_generators()
sage: L[n2, t2] == -n2
True

sage.algebras.lie_algebras.examples.witt(R)

Return the Lie algebra of regular vector fields on $$\CC^{\times}$$.

This is also known as the Witt (Lie) algebra.

EXAMPLES:

sage: lie_algebras.regular_vector_fields(QQ)
The Lie algebra of regular vector fields over Rational Field