Several aspects of fusion rings and fusion rule algebras, and of their manifestations in twodimensional (conformal) field theory, are described: diagonalization and the connection with modular invariance; the presentation in terms of quotients of polynomial rings; fusion graphs; various strategies that allow for a partial classification; and the role of the fusion rules in the conformal bootstrap programme. J. FUCHS, Fusion Rules -The coupling of primary fields of W-algebras in two-dimensional conformal field theory [9-111. (For a few further realizations see section 7 below.) If the axioms of a fusion rule algebra are slightly relaxed, one can also describe: -The multiplication of (classes of) polynomials in any quotient of a polynomial ring, e.g. the ring of chiral primary fields in N = 2 superconformal field theory [12,13]. -Operator products in topological field theory [14,15]. In the present paper the main interest is in the realization of fusion rules in conformal field theory, but to motivate the concept of fusion rings and fusion rule algebras it seems to me most convenient to start with the first example in the above list, i.e. with the decomposition of tensor products of finite-dimensional representations, respectively modules, of a reductive Lie algebra. Thus let g denote a simple Lie algebra (the generalization to arbitrary reductive Lie algebras will be immediate). Any finitedimensional module of g and any tensor product of such modules is fully reducible, i.e. the direct sum of irreducible modules. The finite-dimensional irreducible modules are highest weight modules labelled by a dominant integral highest weight A of g; I denote them by LA, and their Kronecker tensor product and its decomposition into irreducible modules by LA x LA. = @ NAA."" L&.
A"(Here and below I use, for V a vector space and n E Z,,, the short-hand notation nV in place of @a=1 V @ ) with I / ( ' ) Let me recall a few well-known properties of such tensor products: (a) The addition 0 and product x are commutative, associative, and distributive.(b) By definition, the numbers N,,,"" are non-negative integers.(c) The number of highest weight modules LA is infinite; but for fixed A and A', NAAeA" is nonzero only for a finite number of highest weights A". (d) The highest weight module with highest weight A = 0 (the trivial one-dimensional module) acts as the identity, i.e. LA x Lo = LA, or in other words
V.)for any dominant integral highest weight A . (e) To any module LA there exists a unique conjugate module L; which is again a finite-dimensional highest weight module (namely Lfi = LA+, with A' being minus the lowest weight of LA), such that (L;)' = LA, that Lo appears in LA * L; precisely once, i.e. and that the tensor product of conjugate modules is conjugate to the tensor product of the modules themselves, in the sense that (f) The trivial module Lo is self-conjugate. J. FUCHS, Fusion Rules Then commutativity means while associativity is expressed by (F2), J u^i j k N k F = 1 J j . , " 4 k m . ksl kei The positivity axiom (F3...