The Lie algebra can be represented by antisymmetric complex matrices. The compact real form is , naturally represented as the set of antisymmetric real matrices. One way to describe the real subalgebras , aligned with the compact form , is to consider as the set of infinitesimal rotations expressed in Pauli coordinates, i.e., to represent the hyperbolic space on which they act as a Euclidean space whose first coordinates, , are real while the last coordinates are purely imaginary. Writing the matrices of in block form as
To proceed, let us denote by the matrices whose entries are everywhere vanishing except for a block,
on the diagonal. These matrices have the following realisation in terms of the (defined in Equation (6.83)):
Motivated by the dimensional reduction of supergravity, we shall assume , even. We first consider . Then by reordering the coordinates as follows,30. These generators are all orthogonal to each other. Let us denote the elements of the dual basis by , and split them into two subsets: and . The action of the Cartan involution on these generators is very simple,
From Equation (6.179) we also obtain without effort that the set of restricted roots consists of the roots , each of multiplicity one, and the roots , each of multiplicity . These constitute a root system.
Following the same procedure as for the previous case, we obtain a Cartan subalgebra consisting of noncompact generators and compact generators. The corresponding Tits–Satake diagrams are displayed in Figure 39.
The restricted root system is now of type , with long roots of multiplicity one and short roots of multiplicity .
Here the root system is of type , represented by , where the orthonormal vectors again constitute a basis dual to the natural Cartan subalgebra of . Now, and are both assumed even, and we may always suppose . The Cartan involution to be considered acts as previously on the :
on which the Cartan involution has the following action:
The corresponding Tits–Satake diagrams are obtained in the same way as before and are displayed in Figure 40.
When , the restricted root system is again of type , with long roots of multiplicity one and short roots of multiplicity . For , the short roots disappear and the restricted root system is of type, with all roots having multiplicity one.
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 2.0 Germany License.