Abstract. We describe the universal central extension of the three point current algebra sl(2, R) where R = C[t, t −1 , u | u 2 = t 2 + 4t] and construct realizations of it in terms of sums of partial differential operators.
IntroductionIt is well known from the work of C. Kassel and J.L. Loday (see [20], and [21]) that if R is a commutative algebra and g is a simple Lie algebra, both defined over the complex numbers, then the universal central extensionĝ of g ⊗ R is the vector space (g ⊗ R) ⊕ Ω 1 R /dR where Ω 1 R /dR is the space of Kähler differentials modulo exact forms (see [21]). The vector spaceĝ is made into a Lie algebra by defining . We find such a realization in the setting where g = sl(2, C) and R = C[t, t −1 , u|u 2 = t 2 + 4t] is the three point algebra. In Kazhdan and Luszig's explicit study of the tensor structure of modules for affine Lie algebras (see [22] and [23]) the ring of functions regular everywhere except at a finite number of points appears naturally. This algebra M. Bremner gave the name n-point algebra. In particular in the monograph [16, Ch. 12] algebras of the form ⊕ n i=1 g((t − x i )) ⊕ Cc appear in the description of the conformal blocks. These contain the n-point algebras g ⊗ C[(t − x 1 ) −1 , . . . , (t − x N ) −1 ] ⊕ Cc modulo part of the center Ω R /dR. M. Bremner explicitly described the universal central extension of such an algebra in [3].Consider now the Riemann sphere C ∪ {∞} with coordinate function s and fix three distinct points a 1 , a 2 , a 3 on this Riemann sphere. Let R denote the ring of rational functions with poles only in the set {a 1 , a 2 , a 3 }. It is known that the automorphism group P GL 2 (C) of C(s) is simply 3-transitive and R is a subring of C(s), so that R is isomorphic to the ring of rational functions with poles at {∞, 0, 1, a}. Motivated by this isomorphism one sets a = a 4 and here the 4-point, u] where u 2 = t 2 − 2bt + 1 with b a complex number not equal to ±1. Then M. Bremner has shown us that R a ∼ = S b . As the later, being Z 2 -graded, is a cousin to super Lie algebras, and is thus more immediately amendable to the theatrics of conformal field theory. Moreover Bremner has given an explicit description of the universal central extension of g ⊗ R, in terms of ultraspherical 1991 Mathematics Subject Classification. Primary 17B67, 81R10.