Abstract. In this paper, we study 4-dimensional nilpotent complex associative algebras. This is a continuation of the study of the moduli space of 4-dimensional algebras. The non-nilpotent algebras were analyzed in an earlier paper. Even though there are only 15 families of nilpotent 4-dimensional algebras, the complexity of their behavior warranted a separate study, which we give here.
Construction of the algebras by extensionsThe authors and collaborators have been carrying out a construction of moduli spaces of low dimensional complex and real Lie and associative algebras in a series of papers. Our method of constructing the moduli spaces of such algebras is based on the principal that algebras are either simple or can be constructed as extensions of lower dimensional algebras. There is a classical theory of extensions, which was developed by many contributors going back as early as the 1930s. In [2], we gave a description of the theory of extensions of an algebra W by an algebra M . Consider the diagram 0 → M → V → W → 0 of associative K-algebras, so that V = M ⊕ W as a K-vector space, M is an ideal in the algebra V , and W = V /M is the quotient algebra. Suppose that δ ∈ C 2 (W ) = Hom(T 2 (W ), W ) and µ ∈ C 2 (M ) represent the algebra structures on W and M respectively. We can view µ and δ as elements of C 2 (V ). Let T k,l be the subspace of T k+l (V ) given recursively byIf we denote the algebra structure on V by d, we have d = δ + µ + λ + ψ, where λ ∈ C 1,1 and ψ ∈ C 0,2 . Note that in this notation, µ ∈ C 2,0 . Then the condition that d is associative: