Simple extensions of combinatorial structures

Abstract

An interval in a combinatorial structure S is a set I of points which relate to every point from S I in the same way. A structure is simple if it has no proper intervals. Every combinatorial structure can be expressed as an inflation of a simple structure by structures of smaller sizes -- this is called the substitution (or modular) decomposition. In this paper we prove several results of the following type: An arbitrary structure S of size n belonging to a class C can be embedded into a simple structure from C by adding at most f(n) elements. We prove such results when C is the class of all tournaments, graphs, permutations, posets, digraphs, oriented graphs and general relational structures containing a relation of arity greater than 2. The function f(n) in these cases is 2, \lceil log_2(n+1)\rceil, \lceil (n+1)/2\rceil, \lceil (n+1)/2\rceil, \lceil log_4(n+1)\rceil, \lceil \log_3(n+1)\rceil and 1, respectively. In each case these bounds are best possible.