Prime 2-structures

The 2-structures generalize arc-coloured digraphs. They are obtained from a vertex set by considering an equivalence relation between the ordered pairs of distinct vertices. The 2-structures provide a suitable framework to introduce the modular decomposition. A vertex subset of a 2-structure is a module whenever each vertex outside is linked in an equivalent way to all the vertices inside. A 2-structure is prime if it does not admit proper modules with at least 2 elements. Our purpose is to study the possible cardinalities of the prime 2-substructures in a finite or infinite prime 2-structure.