The simple algebra G_2

The simple Lie group G_2 is the stabilizator group of the following (0,3)-tensor ω.

ω = Plus @@ ((Id[7][[#1[[1]]]] ∧Id[7][[#1[[2]]]]) ∧Id[7][[#1[[3]]]] &/@({{1, 2, 7}, {1, 3, 5}, {1, 6, 4}, {2, 6, 3}, {2, 5, 4}, {3, 4, 7}, {5, 6, 7}})) ;

We obtain a basis of the matrix algebra of G_2 using the command glAlgebra.

ShowMat[mats = Algebra[7, Τdα[ω, 0, #1] &]]

The algebra is 14-dimensional. The commutator tensor with respect to the matrix basis above is

ShowAd[ad = 2ad[mats]]

In order to obtain a root system, we obtain a Cartan subalgebra by guessing a regular element. The rank of the algebra is 2, so the element X=e_2+e_13 used below is regular, because the eigenspace of (ad . X)^n is two dimensional for great values of n.

MF[0 = adEigenspace[ad, Id[14][[2]] + Id[14][[13]], 0]]

( {{1, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {-1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0}} )

We transform the basis so that the Cartan subalgebra is spanned by the first two elements.

ShowAd[adC = Τα[ad, 1, T[Join[0, NS[0]]]]]

Unfortunately, our implementation is not good enougth to solve for the roots at this point. The characteristic polynomial is too complicated.


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