New Results on Multipartite Access Structures.

In a multipartite access structure, the set of players is divided into K different classes in such a way that all players of the same class play the same role in the structure. Not many results are known about these structures, when K ≥ 3.Although the total characterisation of ideal multipartite access structures seems a very ambitious goal, we nevertheless take a first step in this direction. On the one hand, we detect some conditions that directly imply that a multipartite structure cannot be ideal. On the other hand, we introduce a new strategy which helps to prove that a multipartite access structure is ideal, and we apply this strategy to three wide families of multipartite access structures.