“…The main motivation for the present paper stems from the recent activities on categorification of representations of various algebras, see, in particular, [CR,FKS,HS,HK,KMS1,Lau,KL,MM,MS1,MS3,MS4,R,Zh1,Zh2], the reviews [KMS2,Ma2] and references therein. In these articles one could find several results of the following kind: given a field k, an associative k-algebra Λ with a fixed generating set {a i }, and a Λ-module M, one constructs a categorification of M, that is an abelian category C and exact endofunctors {F i } of C such that the following holds: The Grothendieck group [C] of C (with scalars extended to an appropriate field) is isomorphic to M as a vector space and the functor F i induces on [C] the action of a i on M. Typical examples of algebras, for which categorifications of certain modules are constructed, include group algebras of Weyl groups, Hecke algebras, Schur algebras and enveloping algebras of some Lie algebras.…”