Unavoidable doubly connected large graphs Original Research Article

A connected graph is doubly connected if its complement is also connected. The following Ramsey-type theorem is proved in this paper. There exists a function h(n), defined on the set of integers exceeding three, such that every doubly connected graph on at least h(n) vertices must contain, as an induced subgraph, a doubly connected graph, which is either one of the following graphs or the complement of one of the following graphs: (1) Pn, a path on n vertices; (2) K1,ns, the graph obtained from K1,n by subdividing an edge once; (3) K2,n⧹e, the graph obtained from K2,n by deleting an edge; (4) K2,n+, the graph obtained from K2,n by adding an edge between the two degree-n vertices x1 and x2, and a pendent edge at each xi. Two applications of this result are also discussed in the paper.