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

  • The symplectic derivation Lie algebra

  • The rank two Heisenberg Virasoro algebra

See also sage.algebras.lie_algebras.virasoro.LieAlgebraRegularVectorFields and sage.algebras.lie_algebras.virasoro.VirasoroAlgebra for other examples.

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.

See also

LieAlgebraRegularVectorFields

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.

A classical description of this as a matrix Lie algebra is the set of all anti-symmetric \(n \times n\) matrices. However, the implementation here uses a different bilinear form for the Lie group and follows the description in Chapter 8 of [HK2002]. See sage.algebras.lie_algebras.classical_lie_algebra.so for a precise description.

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.su(R, n, representation='matrix')#

The Lie algebra \(\mathfrak{su}_n\).

The Lie algebra \(\mathfrak{su}_n\) is the compact real form of the type \(A_{n-1}\) Lie algebra and is finite-dimensional. As a matrix Lie algebra, it is given by the set of all \(n \times n\) skew-Hermitian matrices with trace 0.

INPUT:

  • R – the base ring

  • n – the size of the matrix

  • representation – (default: 'matrix') can be one of the following:

    • 'bracket' - use brackets and the Chevalley basis

    • 'matrix' - use matrices

EXAMPLES:

We construct \(\mathfrak{su}_2\), where the default is as a matrix Lie algebra:

sage: su2 = lie_algebras.su(QQ, 2)
sage: E,H,F = su2.basis()
sage: E.bracket(F) == 2*H
True
sage: H.bracket(E) == 2*F
True
sage: H.bracket(F) == -2*E
True

Since \(\mathfrak{su}_n\) is the same as the type \(A_{n-1}\) Lie algebra, the bracket is the same as sl():

sage: su2 = lie_algebras.su(QQ, 2, representation='bracket')
sage: su2 is lie_algebras.sl(QQ, 2, representation='bracket')
True
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.

See also

LieAlgebraRegularVectorFields

EXAMPLES:

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