In this paper we present the extension of the copula approach to ag-gregation functions. In fact we want to focus on a class of aggregation functions and present them in the multi linear form with marginal copu-lae. Moreover we will define also the joint aggregation density function.

This paper deals with the construction of RET (relevancy transformation) operators for fuzzy systems. The notion of pseudo-duality is introduced to obtain new RET operators from old ones and, t-norms, tconorms, nullnorms and uninorms are used to construct this kind of operators.

Following the ideas of stronger forms of monotonicity for unary real functions and for capacities, k-monotone and strongly kmonotone aggregation functions are introduced and discussed. In the special case k = 2 also some applications are given.