GroupActions[LiesThirdTheorem] - find a Lie algebra of pointwise independent vector fields with prescribed structure equations (solvable algebras only)
Calling Sequences
LiesThirdTheorem(Alg, M, option)
LiesThirdTheorem(A, M)
Parameters
Alg - a Maple name or string, the name of an initialized Lie algebra g
M - a Maple name or string, the name of an initialized manifold with the same dimension as that of g
option - with output = "forms" the dual 1-forms (Maurer-Cartan forms) are returned
A - a list of square matrices, defining a matrix Lie algebra
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Description
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Let g be an n-dimensional Lie algebra with structure constants C. Then Lie's Third Theorem (see, for example, Flanders, page 108) asserts that there is, at least locally, a Lie algebra of n pointwise independent vector fields Gamma on an n-dimensional manifold M with structure constants C.
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The command LiesThirdTheorem(Alg, M) produces a globally defined Lie algebra of vector fields Gamma in the special case that g is solvable. More general cases will be handled in subsequent versions of the DifferentialGeometry package.
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The command LiesThirdTheorem(A, M) produces a globally defined matrix of 1-forms (Maurer-Cartan forms) in the special case that the list of matrices A defines a solvable Lie algebra.
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The command LiesThirdTheorem is part of the DifferentialGeometry:-GroupActions package. It can be used in the form LiesThirdTheorem(...) only after executing the commands with(DifferentialGeometry) and with(GroupActions), but can always be used by executing DifferentialGeometry:-GroupActions:-LiesThirdTheorem(...).
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Examples
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Example 1.
We obtain a Lie algebra from the DifferentialGeometry library using the Retrieve command and initialize it.
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We define a manifold M of dimension 4 (the same dimension as the Lie algebra).
Alg1 >
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M1 >
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M1 >
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We calculate the structure equations for the Lie algebra of vector fields Gamma1 and check that these structure equations coincide with those for Alg1.
M1 >
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Example 2.
We re-work the previous example in a more complicated basis. In this basis the adjoint representation is not upper triangular, in which case LiesThirdTheorem first calls the program SolvableRepresentation to find a basis for the algebra in which the adjoint representation is upper triangular. (Remark: It is almost always useful, when working with solvable algebras, to transform to a basis where the adjoint representation is upper triangular.)
M1 >
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![_DG([["LieAlgebra", Alg2, [4]], [[[1, 2, 1], -1], [[1, 2, 2], -1], [[1, 2, 3], 1], [[1, 2, 4], -1], [[1, 3, 1], -1], [[1, 3, 2], -1], [[1, 3, 3], -1], [[1, 4, 1], -1], [[1, 4, 2], -1], [[1, 4, 4], -1], [[2, 3, 1], 1], [[2, 3, 2], 1], [[2, 3, 3], 1], [[2, 4, 1], 1], [[2, 4, 2], 1], [[2, 4, 4], 1]]])](/support/helpjp/helpview.aspx?si=6645/file05748/math179.png)
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Alg2 >
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Alg2 >
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![[_DG([["vector", M1, []], [[[1], x-y], [[2], y+z], [[3], z], [[4], 1]]]), _DG([["vector", M1, []], [[[1], -x+y], [[2], 1-y-z], [[3], -z], [[4], -1]]]), _DG([["vector", M1, []], [[[3], 1]]]), _DG([["vector", M1, []], [[[1], -1], [[3], 1]]])]](/support/helpjp/helpview.aspx?si=6645/file05748/math193.png)
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M1 >
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![_DG([["LieAlgebra", Alg2a, [4]], [[[1, 2, 1], -1], [[1, 2, 2], -1], [[1, 2, 3], 1], [[1, 2, 4], -1], [[1, 3, 1], -1], [[1, 3, 2], -1], [[1, 3, 3], -1], [[1, 4, 1], -1], [[1, 4, 2], -1], [[1, 4, 4], -1], [[2, 3, 1], 1], [[2, 3, 2], 1], [[2, 3, 3], 1], [[2, 4, 1], 1], [[2, 4, 2], 1], [[2, 4, 4], 1]]])](/support/helpjp/helpview.aspx?si=6645/file05748/math200.png)
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Example 3.
Here is an example where one of the adjoint matrices has complex eigenvalues. The Lie algebra contains parameters p and b.
M1 >
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M1 >
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Alg3 >
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Alg3 >
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M3 >
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![[_DG([["vector", M3, []], [[[1], 1]]]), _DG([["vector", M3, []], [[[1], -z], [[2], 1]]]), _DG([["vector", M3, []], [[[3], 1]]]), _DG([["vector", M3, []], [[[4], 1]]]), _DG([["vector", M3, []], [[[1], (1/2)*z^2-(1/2)*y^2+2*p*x], [[2], y*p-z], [[3], z*p+y], [[4], b*u], [[5], 1]]])]](/support/helpjp/helpview.aspx?si=6645/file05748/math242.png)
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M3 >
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Example 4.
We calculate the Maurer-Cartan matrix of 1-forms for a solvable matrix algebra, namely the matrices defining the adjoint representation for Alg1 from Example 1.
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Note that the elements of this matrix
coincide with the appropriate linear combinations of the forms in the list from Example 1.
Alg1 >
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Alg1 >
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Alg1 >
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Alg1 >
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