Abstract. We show that the rank 10 hyperbolic Kac-Moody algebra E10 contains every simply laced hyperbolic Kac-Moody algebra as a Lie subalgebra. Our method is based on an extension of earlier work of Feingold and Nicolai.
IntroductionSince their discovery [9,15] by Kac and Moody, Kac-Moody algebras have been playing increasingly important roles in diverse subfields of mathematics and physics. The affine Kac-Moody algebras are by now as well understood as the finite dimensional simple Lie algebras classified by Cartan and Killing. Indefinite type Kac-Moody algebras however remain a notoriously intractable part of the theory. In spite of much work in this direction (see [7,11] and the references in [8]), obtaining detailed information about the structure of these Lie algebras seems out of reach at present. Most of the available results concern the subclass of hyperbolic Kac-Moody algebras. Such algebras only exist in ranks 2-10 and can be completely classified [16] (see also [5] for some missing diagrams). Among these, the algebra E 10 has been singled out for its relevance to string theory and has received much attention in recent times (e.g: [1][2][3][4][12][13][14]).In [8], Feingold and Nicolai studied subalgebras of hyperbolic Kac-Moody algebras. They showed that the rank 3 hyperbolic Kac-Moody algebra F = HA (1) 1 contains every rank 2 hyperbolic Kac-Moody algebra with symmetric generalized Cartan matrix; in fact F was also shown to contain an infinite series of indefinite type Kac-Moody algebras. Analogously, it was shown that there are infinitely many inequivalent Kac-Moody algebras of indefinite type that occur as Lie subalgebras of E 10 .This motivates the main question of this letter: which Kac-Moody algebras of hyperbolic type occur as Lie subalgebras of E 10 ? To answer this question, we extend the method of Feingold-Nicolai and formulate some general principles for constructing Lie subalgebras. These principles are then used to prove our main result: Every simply laced hyperbolic Kac-Moody algebra occurs as a Lie subalgebra of E 10 . This statement can be viewed as further evidence of the distinguished role played by E 10 in the family of 2000 Mathematics Subject Classification. 17B67.