LieAlgebras[SolvableRepresentation] - given a representation of a solvable algebra, find a basis for the representation space in which the representation matrices are upper triangular matrices
Calling Sequences
SolvableRepresentation( ρ, options)
SolvableRepresentation(Alg, options)
Parameters
ρ - a representation of a solvable Lie algebra 𝔤 on a vector space V
alg - a string or name, the name of a initialized solvable Lie algebra
options - the keyword argument output = O, where O is a list with members "NewBasis", ChangeOfBasisMatrix", "TransformedMatrices", "Partition"; the keyword argument fieldextension = I
Description
Examples
Let rho: 𝔤 → glVbe a representation of a solvable Lie algebra 𝔤 on a vector space V. A corollary of Lie's fundamental theorem for solvable Lie algebras (see RepresentationEigenvector) implies that there always exists a basis (possibly complex) for V such that the matrix representation of ρxis upper triangular for all x ∈𝔤.
The program SolvableRepresentation(rho) uses the program RepresentationEigenvector to construction such a basis. In the case when the RepresentationEigenvector program returns a complex eigenvector (with associated complex eigenvalue a + bI), the matrix representation will not be upper triangular but will contain the matrix ab−ba on the diagonal (similar to the real Jordan form of a matrix).
For the second calling sequence, the program SolvableRepresentation is applied to the adjoint representation of the algebra Alg.
The output is a 4-element sequence. The 1st element is a new basis ℬ forV in which the representation is upper triangular, the 2nd element is the change of basis matrix, the 3rd element is the representation in the new basis. The 4th element P gives the partition defining the size of the diagonal block matrices. If P = 1.. n1, n1+1 .. n2, n2+1 .. n3, ... , then the subspaces ℬ1, ..., n1, ℬ1, ..., n2, ℬ1, ..., n3 are ρ−invariant subspaces. If, for example, P = 1.. 1, 2.. 2 , 3.. 3, then all the eigenvectors calculated by RepresentationEigenvector are real. If C = 1..1, 2..3 then the vectors ℬ2 and ℬ3 are the real and imaginary parts of a complex eigenvector. The precise form of the output can be specified by the user with the keyword argument output = O, where O is a list with members "NewBasis", ChangeOfBasisMatrix", "TransformedMatrices", "Partition".
With the option fieldextension = I, a complex basis will be returned (if needed) which puts the representation into upper triangular form.
with⁡DifferentialGeometry:with⁡LieAlgebras:with⁡Library:
Example 1.
We define a 5-dimensional representation of a 3-dimensional solvable Lie algebra.
L≔_DG⁡LieAlgebra,alg1,3,1,2,2,1,2,3,2,1
L:=e1,e2=e2,e2,e3=e2
DGsetup⁡L:
DGsetup⁡x1,x2,x3,x4,x5,V1:
M≔map⁡Matrix,8,8,0,0,0,−1,5,6,0,0,0,−2,2,4,0,0,0,−3,−1,2,0,0,0,−4,−4,8,16,0,0,0,−1,4,12,0,0,0,−2,0,8,0,0,0,−3,−4,4,0,0,0,−4,−8,−4,−8,0,0,0,1,−1,−6,0,0,0,2,2,−4,0,0,0,3,5,−2,0,0,0,4,8:
ρ1≔Representation⁡alg1,V1,M
We find a new basis for the representation space in which the matrices are all upper triangular.
B1,P1,newrho,Part1≔SolvableRepresentation⁡ρ1
To verify this result we use the ChangeRepresentationBasis command to change basis in the representation space.
ChangeRepresentationBasis⁡ρ1,B1,V1
Example 2.
We define a 6-dimensional representation of a 3-dimensional solvable Lie algebra.
L2≔_DG⁡LieAlgebra,Alg2,3,1,3,2,−1,1,3,1,3,2,3,1,1,2,3,2,3
L2:=e1,e3=−e2+3⁢e1,e2,e3=e1+3⁢e2
DGsetup⁡L2:
DGsetup⁡x1,x2,x3,x4,x5,x6,V2:
M≔map⁡Matrix,0,0,0,0,0,0,0,0,0,0,0,0,−3,1,0,0,0,0,0,0,0,0,0,0,0,−3,0,1,0,0,0,0,−2⋅3,0,2,0,0,0,0,0,0,0,0,0,0,0,0,0,−1,−3,0,0,0,0,0,0,0,0,0,0,0,−1,0,−3,0,0,0,0,−2,0,−2⋅3,0,2⋅3,−2,0,0,0,0,1,2⋅3,0,−1,0,0,0,0,3,0,−1,0,0,2,0,2⋅3,0,0,0,0,1,0,3,0,0,0,0,0,0,0:
ρ2≔Representation⁡Alg2,V2,M
In this example some of the eigenvectors found by the RepresentationEigenvector program are complex and it is not possible to find a real basis in which the representation is upper triangular.
Query⁡ρ2,Representation
true
B2,P2,newrho,Part2≔SolvableRepresentation⁡ρ2
ChangeRepresentationBasis⁡ρ2,B2,V2
To obtain an upper triangular representation with respect to a complex basis, use the optional argument fieldextension = I.
B3≔SolvableRepresentation⁡ρ2,fieldextension=I,output=NewBasis
B3:=D_x6,D_x3−I⁢D_x5,D_x3+I⁢D_x5,D_x1−I⁢D_x2−D_x4,D_x1+D_x4,D_x1+I⁢D_x2−D_x4
ChangeRepresentationBasis⁡ρ2,B3,V2
Example 3.
If the name of an algebra is passed to the program SolvableRepresentation, then the assumed representation is the adjoint representation of the algebra (or current frame).
L3≔_DG⁡LieAlgebra,Alg3,5,1,2,1,−1,1,2,5,1,1,3,1,1,1,3,5,−1,1,4,1,2,1,4,2,1,1,4,3,1,2,3,1,−1,2,3,5,1,2,4,3,−1,2,5,1,1,2,5,5,−1,3,4,3,1,3,4,5,−1,3,5,1,−1,3,5,5,1,4,5,2,−1,4,5,3,−1,4,5,5,−2
L3:=e1,e2=−e1+e5,e1,e3=e1−e5,e1,e4=2⁢e1+e2+e3,e2,e3=−e1+e5,e2,e4=−e3,e2,e5=e1−e5,e3,e4=e3−e5,e3,e5=−e1+e5,e4,e5=−e2−e3−2⁢e5
DGsetup⁡L3:
The adjoint representation of this algebra is not upper triangular.
Adjoint⁡
B≔SolvableRepresentation⁡Alg3,output=NewBasis
B:=e2+e3+e5,e1−e5,e1,e2,e4
L4≔LieAlgebraData⁡B,Alg4
L4:=e1,e5=e1,e2,e5=2⁢e2,e3,e4=−e2,e3,e5=e1+e2+e3,e4,e5=−e1−e2+e3+e4
DGsetup⁡L4:
Now in this new basis the adjoint representation is upper triangular.
Example 4.
An example with complex eigenvalues.
L≔_DG⁡LieAlgebra,Alg5,5,1,2,1,5,1,2,2,−5,1,2,3,−3,1,2,5,−2,1,3,1,−1,1,3,2,1,1,3,3,−1,1,3,5,2,1,4,1,4,1,4,2,−3,1,4,3,−3,1,4,4,−1,1,4,5,−1,1,5,1,−4,1,5,2,5,1,5,3,3,1,5,4,−1,1,5,5,1,2,3,1,−2,2,3,2,2,2,3,5,2,2,4,2,1,2,4,4,−1,2,5,1,−4,2,5,2,4,2,5,3,3,2,5,5,1,3,4,1,1,3,4,2,−1,3,4,5,−1,3,5,1,−1,3,5,2,2,3,5,3,1,3,5,4,−1,4,5,1,−3,4,5,2,3,4,5,3,3
L:=e1,e2=5⁢e1−5⁢e2−3⁢e3−2⁢e5,e1,e3=−e1+e2−e3+2⁢e5,e1,e4=4⁢e1−3⁢e2−3⁢e3−e4−e5,e1,e5=−4⁢e1+5⁢e2+3⁢e3−e4+e5,e2,e3=−2⁢e1+2⁢e2+2⁢e5,e2,e4=e2−e4,e2,e5=−4⁢e1+4⁢e2+3⁢e3+e5,e3,e4=e1−e2−e5,e3,e5=−e1+2⁢e2+e3−e4,e4,e5=−3⁢e1+3⁢e2+3⁢e3
B1,C1≔SolvableRepresentation⁡Alg5,output=NewBasis,Partition
B1,C1:=e1−e2−e3,e1−e4−e5,e2−e4,e1,e2,1..1,2..3,4..5
L2≔LieAlgebraData⁡B1,Alg6
L2:=e1,e4=4⁢e1,e1,e5=3⁢e1,e2,e4=2⁢e3,e2,e5=e2,e3,e4=−e2+2⁢e3,e3,e5=e3,e4,e5=3⁢e1+2⁢e2−2⁢e3
In this new basis the adjoint representation is upper triangular except for a 2x2 "complex" block on the diagonal for ad(e4).
Adjoint⁡L2
We rerun this example with the option fieldextension = I
B3≔SolvableRepresentation⁡Alg5,fieldextension=I,output=NewBasis
B3:=e1−e2−e3,e1−1+I⁢e2+I⁢e4−e5,e1−1−I⁢e2−I⁢e4−e5,e1,e2
L3≔LieAlgebraData⁡B3,Alg7
L3:=e1,e4=4⁢e1,e1,e5=3⁢e1,e2,e4=1+I⁢e2,e2,e5=e2,e3,e4=1−I⁢e3,e3,e5=e3,e4,e5=3⁢e1+e2+e3
Adjoint⁡L3
Example 5.
Let rho:𝔤 → V be a representation of a nilpotent Lie algebra 𝔤 on a vector space V. The representation is called a nilrepresentation if each matrix A=ρx is nilpotent, that is Ak=0 for some k. Engel's theorem (see, for example, Fulton and Harris, page 125 or Varadarajan, page 189) asserts that if rho is a nilrepresentation, then there is a basis for V for which all the representation matrices are strictly upper triangular.
L5≔_DG⁡LieAlgebra,Alg5,6,1,2,2,1,1,2,3,1,1,2,4,−1,1,2,5,1,1,3,3,−12,1,3,5,12,1,3,6,−12,1,4,2,1,1,4,3,1,1,4,4,−1,1,4,5,1,1,5,3,12,1,5,5,−12,1,5,6,12,1,6,3,1,1,6,5,−1,1,6,6,1,2,3,3,−12,2,3,5,−12,2,3,6,−12,2,4,5,−1,2,6,3,12,2,6,5,12,2,6,6,12,3,4,5,1,4,6,5,1
L5:=e1,e2=e2+e3−e4+e5,e1,e3=−12⁢e3+12⁢e5−12⁢e6,e1,e4=e2+e3−e4+e5,e1,e5=12⁢e3−12⁢e5+12⁢e6,e1,e6=e3−e5+e6,e2,e3=−12⁢e3−12⁢e5−12⁢e6,e2,e4=−e5,e2,e6=12⁢e3+12⁢e5+12⁢e6,e3,e4=e5,e4,e6=e5
DGsetup⁡L5:
DGsetup⁡x1,x2,x3,x4,V5:
M5≔map⁡Matrix,−5,−9,10,−4,−4,−7,8,−3,−5,−9,10,−4,3,5,−6,2,−8,−12,14,−6,−5,−8,9,−4,−9,−14,16,−7,0,0,0,0,−1,−2,2,−1,0,0,0,0,0,0,0,0,1,2,−2,1,−5,−8,9,−4,0,0,0,0,−5,−8,9,−4,−5,−8,9,−4,−1,−2,2,−1,0,0,0,0,−1,−2,2,−1,−1,−2,2,−1,−2,−4,4,−2,−2,−4,4,−2,−3,−6,6,−3,0,0,0,0:
ρ≔Representation⁡Alg5,V5,M5
Check that Alg5 is a nilpotent algebra, that rho is a representation, and that rho is a nilrepresentation.
Query⁡Alg5,Nilpotent
Query⁡ρ,Representation
Query⁡ρ,NilRepresentation
B≔SolvableRepresentation⁡Alg5,output=NewBasis
B:=e2−e6,e3+e6,e4,e5,e1,e2
L5a≔LieAlgebraData⁡B,Alg5a
L5a:=e1,e5=−e1+e3−2⁢e4,e1,e6=12⁢e2+12⁢e4,e2,e5=−12⁢e2+12⁢e4,e3,e5=−e1−e2+e3−e4,e3,e6=e4,e4,e5=−12⁢e2+12⁢e4,e5,e6=e1+e2−e3+e4
In this new basis the ad matrices are all nilpotent.
Adjoint⁡L5a
See Also
DifferentialGeometry
Library
LieAlgebras
Adjoint
ChangeRepresentationBasis
Query
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