The explicit appearance of infinite crystallographic Coxeter groups in the billiard limit suggests that gravitational theories might be invariant under a huge symmetry described by Lorentzian Kac–Moody algebras (defined in Section 4.1). Indeed, there is an intimate connection between crystallographic Coxeter groups and Kac–Moody algebras. This connection might be familiar in the finite case. For instance, it is well known that the finite symmetry group of the equilateral triangle (isomorphic to the group of permutations of 3 objects) and the corresponding hexagonal pattern of roots are related to the finite-dimensional Lie algebra (or ). The group is in fact the Weyl group of (see Section 4.7).
This connection is not peculiar to the Coxeter group but is generally valid: Any crystallographic Coxeter group is the Weyl group of a Kac–Moody algebra traditionally denoted in the same way (see Section 4.7). This is the reason why it is expected that the Coxeter groups might signal a bigger symmetry structure. And indeed, there are indications that this is so since, as we shall discuss in Section 9, an attempt to reformulate the gravitational Lagrangians in a way that makes the conjectured symmetry manifest yields intriguing results.
The purpose of this section is to develop the mathematical concepts underlying Kac–Moody algebras and to explain the connection between Coxeter groups and Kac–Moody algebras. How this is relevant to gravitational theories will be discussed in Section 5.
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