E6 Next: E7 Up: Exceptional Lie Algebras Previous: The Magic Square

## 4.4 We begin with the 78-dimensional exceptional Lie group . As we mentioned in Section 3.4, there is a nice description of a certain noncompact real form of as the group of collineations of , or equivalently, the group of determinant-preserving linear transformations of . But before going into these, we consider the magic square constructions of the Lie algebra . Vinberg's construction gives Tits' construction, which is asymmetrical, gives and also The Barton-Sudbery construction gives We can use any of these to determine the dimension of . For example, we have Starting from the Barton-Sudbery construction and using the concrete descriptions of and from equation (3), we obtain Using equation (4.2), we may rewrite this as and it turns out that the summand is actually a Lie subalgebra of . This result can also be found in Adams' book , phrased as follows: In fact, he describes the bracket in in terms of natural operations involving and its spinor representation . The funny-looking factor of comes from the fact that this spinor representation is complex. The bracket of an element of and an element of is another element of , defined using the obvious action of on this complex vector space.

If we define to be the simply connected group with Lie algebra , it follows from results of Adams that the subgroup generated by the Lie subalgebra is isomorphic to . This lets us define the bioctonionic projective plane by and conclude that the tangent space at any point of this manifold is isomorphic to .

Since is compact, we can put an -invariant Riemannian metric on the bioctonionic projective plane by averaging any metric with respect to the action of this group. It turns out  that the isometry group of this metric is exactly , so we have It follows that Summarizing, we have 6 octonionic descriptions of :

Theorem 6.   The compact real form of is given by where in each case the Lie bracket of is built from natural bilinear operations on the summands.

The smallest nontrivial representations of are 27-dimensional: in fact it has two inequivalent representations of this dimension, which are dual to one another. Now, the exceptional Jordan algebra is also 27-dimensional, and in 1950 this clue led Chevalley and Schafer  to give a nice description of as symmetries of this algebra. These symmetries do not preserve the product, but only the determinant.

More precisely, the group of determinant-preserving linear transformations of turns out to be a noncompact real form of . This real form is sometimes called , because its Killing form has signature . To see this, note that any automorphism of preserves the determinant, so we get an inclusion This means that is a compact subgroup of . In fact it is a maximal compact subgroup, since if there were a larger one, we could average a Riemannian metric group on with respect to this group and get a metric with an isometry group larger than , but no such metric exists. It follows that the Killing form on the Lie algebra is negative definite on its 52-dimensional maximal compact Lie algebra, and positive definite on the complementary 26-dimensional subspace, giving a signature of .

We saw in Section 3.4 that the projective plane structure of can be constructed starting only with the determinant function on the vector space . It follows that acts as collineations on , that is, line-preserving transformations. In fact, the group of collineations of is precisely : Moreover, just as the group of isometries of fixing a specific point is a copy of , the group of collineations fixing a specific point is . This fact follows with some work starting from equation (3.4), and it gives us a commutative square of inclusions: where the groups on the top are maximal compact subgroups of those on the bottom. Thus in a very real sense, is to 9-dimensional Euclidean geometry as is to 10-dimensional Lorentzian geometry.

Next: E7 Up: Exceptional Lie Algebras Previous: The Magic Square