IsotropySubalgebra - Maple Help
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GroupActions[IsotropySubalgebra] - find the infinitesimal isotropy subalgebra of a Lie algebra of vector fields and the representation of the isotropy subalgebra on the tangent space

Calling Sequences

IsotropySubalgebra(Gamma, p, option)

Parameters

Gamma     - a list of vector fields on a manifold $M$

p         - a list of equations  specifying the coordinates of point

option    - the optional argument output = O, where O is a list containing the keywords "Vector", "Representation", and/or the name of an initialized abstract algebra for the Lie algebra of vector fields Gamma.

Description

 • Let be a Lie algebra of vector fields on a manifold and letThe isotropy subalgebra  of the Lie algebra of vector fields at the point is defined by . The Lie bracket of vector fields defines a natural representation of ${\mathrm{Γ}}_{p}$ on the tangent space  by  for  , and $\stackrel{‾}{Y}$ any vector field on $M$ such that . The representation $\mathrm{ρ}$ is called the linear isotropy representation.
 • IsotropySubalgebra(Gamma, p) returns a list of vectors whose span defines the isotropy subalgebra ${\mathrm{\Gamma }}_{p}$as a subalgebra of  $\mathrm{Γ}$.
 • With output = ["Vector", "Representation"], two lists are returned. The first is a list of vectors giving the isotropy subalgebra ${\mathrm{Γ}}_{p}$as a subalgebra of  and the second is the list of matrices defining the linear isotropy representation with respect to the standard basis for ${T}_{p}M$.
 • Let algname be the name of the abstract Lie algebra created from $\mathrm{Γ}$. With output = ["Vector", algname], the second list returned gives the isotropy subalgebra as a subalgebra of the abstract Lie algebra $\mathrm{𝔤}$.
 • The command IsotropySubalgebra is part of the DifferentialGeometry:-GroupActions package.  It can be used in the form IsotropySubalgebra(...) only after executing the commands with(DifferentialGeometry) and with(GroupActions), but can always be used by executing DifferentialGeometry:-GroupActions:-IsotropySubalgebra(...).

Examples

 > $\mathrm{with}\left(\mathrm{DifferentialGeometry}\right):$$\mathrm{with}\left(\mathrm{GroupActions}\right):$$\mathrm{with}\left(\mathrm{Library}\right):$$\mathrm{with}\left(\mathrm{LieAlgebras}\right):$

Example 1.

We use the Retrieve command to obtain a Lie algebra of vector fields in the paper by Gonzalez-Lopez, Kamran, and Olver from the DifferentialGeometry Library. We compute the isotropy subalgebra and isotropy representation at the points  and

 > $\mathrm{DGsetup}\left(\left[x,y\right],M\right):$
 M > $G≔\mathrm{Retrieve}\left("Gonzalez-Lopez",1,\left[5\right],\mathrm{manifold}=M\right)$
 ${G}{:=}\left[{\mathrm{D_x}}{,}{\mathrm{D_y}}{,}{\mathrm{D_x}}{}{x}{-}{\mathrm{D_y}}{}{y}{,}{y}{}{\mathrm{D_x}}{,}{x}{}{\mathrm{D_y}}\right]$ (2.1)
 M > $L≔\mathrm{LieAlgebraData}\left(G,\mathrm{Alg1}\right)$
 ${L}{:=}\left[\left[{\mathrm{e1}}{,}{\mathrm{e3}}\right]{=}{\mathrm{e1}}{,}\left[{\mathrm{e1}}{,}{\mathrm{e5}}\right]{=}{\mathrm{e2}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e3}}\right]{=}{-}{\mathrm{e2}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e4}}\right]{=}{\mathrm{e1}}{,}\left[{\mathrm{e3}}{,}{\mathrm{e4}}\right]{=}{-}{2}{}{\mathrm{e4}}{,}\left[{\mathrm{e3}}{,}{\mathrm{e5}}\right]{=}{2}{}{\mathrm{e5}}{,}\left[{\mathrm{e4}}{,}{\mathrm{e5}}\right]{=}{-}{\mathrm{e3}}\right]$ (2.2)
 M > $\mathrm{DGsetup}\left(L\right)$
 ${\mathrm{Lie algebra: Alg1}}$ (2.3)
 Alg1 > $\mathrm{MultiplicationTable}\left("LieTable"\right)$

We illustrate some different possible outputs from the IsotropySubalgebra program.

 Alg1 > $\mathrm{Iso1}≔\mathrm{IsotropySubalgebra}\left(G,\left[x=0,y=0\right]\right)$
 ${\mathrm{Iso1}}{:=}\left[{\mathrm{D_x}}{}{x}{-}{\mathrm{D_y}}{}{y}{,}{y}{}{\mathrm{D_x}}{,}{x}{}{\mathrm{D_y}}\right]$ (2.4)
 M > $\mathrm{Iso1},\mathrm{A1}≔\mathrm{IsotropySubalgebra}\left(G,\left[x=0,y=0\right],\mathrm{output}=\left["Vector",\mathrm{Alg1}\right]\right)$
 ${\mathrm{Iso1}}{,}{\mathrm{A1}}{:=}\left[{\mathrm{D_x}}{}{x}{-}{\mathrm{D_y}}{}{y}{,}{y}{}{\mathrm{D_x}}{,}{x}{}{\mathrm{D_y}}\right]{,}\left[{\mathrm{e3}}{,}{\mathrm{e4}}{,}{\mathrm{e5}}\right]$ (2.5)
 Alg1 > $\mathrm{Iso1},\mathrm{A1},\mathrm{S1}≔\mathrm{IsotropySubalgebra}\left(G,\left[x=0,y=0\right],\mathrm{output}=\left["Vector",\mathrm{Alg1},"Representation"\right]\right)$
 Alg1 > $\mathrm{A1}≔\mathrm{IsotropySubalgebra}\left(G,\left[x=0,y=0\right],\mathrm{output}=\left[\mathrm{Alg1}\right]\right)$
 ${\mathrm{A1}}{:=}\left[{\mathrm{e3}}{,}{\mathrm{e4}}{,}{\mathrm{e5}}\right]$ (2.6)
 Alg1 > $\mathrm{Iso2},\mathrm{A2},\mathrm{S2}≔\mathrm{IsotropySubalgebra}\left(G,\left[x=1,y=1\right],\mathrm{output}=\left["Vector",\mathrm{Alg1},"Representation"\right]\right)$

Note that the vectors in Iso2 all vanish at

It is apparent from the multiplication table that the pair Alg1, S1 is a symmetric pair with respect to the complementary subspace. We can check this with the command Query/"SymmetricPair".

 Alg1 > $\mathrm{Query}\left(\mathrm{A1},\left[\mathrm{e1},\mathrm{e2}\right],"SymmetricPair"\right)$
 ${\mathrm{true}}$ (2.7)

The linear isotropy representation can be converted to a representation.

 Alg1 > $\mathrm{L2}≔\mathrm{LieAlgebraData}\left(\mathrm{A1},\mathrm{iso1}\right)$
 ${\mathrm{L2}}{:=}\left[\left[{\mathrm{e1}}{,}{\mathrm{e2}}\right]{=}{-}{2}{}{\mathrm{e2}}{,}\left[{\mathrm{e1}}{,}{\mathrm{e3}}\right]{=}{2}{}{\mathrm{e3}}{,}\left[{\mathrm{e2}}{,}{\mathrm{e3}}\right]{=}{-}{\mathrm{e1}}\right]$ (2.8)
 Alg1 > $\mathrm{DGsetup}\left(\mathrm{L2}\right)$
 ${\mathrm{Lie algebra: iso1}}$ (2.9)
 iso1 > $\mathrm{ρ}≔\mathrm{Representation}\left(\mathrm{iso1},M,\mathrm{S1}\right)$
 iso1 > $\mathrm{Query}\left(\mathrm{ρ},"Representation"\right)$
 ${\mathrm{true}}$ (2.10)