Semicanonical bases and preprojective algebras

We study the multiplicative properties of the dual of Lusztig's semicanonical basis. The elements of this basis are naturally indexed by the irreducible components of Lusztig's nilpotent varieties, which can be interpreted as varieties of modules over preprojective algebras. We prove that the product of two dual semicanonical basis vectors ρZ^′ and ρZ^″ is again a dual semicanonical basis vector provided the closure of the direct sum of the corresponding two irreducible components Z^′ and Z^″ is again an irreducible component. It follows that the semicanonical basis and the canonical basis coincide if and only if we are in Dynkin type An with n?4. Finally, we provide a detailed study of the varieties of modules over the preprojective algebra of type A₅. We show that in this case the multiplicative properties of the dual semicanonical basis are controlled by the Ringel form of a certain tubular algebra of type (6,3,2) and by the corresponding elliptic root system of type .