The total detour monophonic number of a graph.

Abstract

For a connected graph G = (V, E) of order at least two, a chord of a path P is an edge joining two non-adjacent vertices of P. A path P is called a monophonic path if it is a chordless path. A longest x – y monophonic path is called an x – y detour monophonic path. A set S of vertices of G  is a detour monophonic set of G if each vertex v of G lies on an x - y detour monophonic path for some x and y in S. The minimum cardinality of a detour monophonic set of G is the detour monophonic number of G and is denoted by dm(G). A total detour monophonic set of a graph G is a detour monophonic set S such that the subgraph induced by S has no isolated vertices. The minimum cardinality of a total detour monophonic set of G is the total detour monophonic number of G and is denoted by dm_t(G). A total detour monophonic set of cardinality dm_t(G) is called a dm_t-set of G. We determine bounds for it and characterize graphs which realize the lower bound. It is shown that for positive integers r, d and k ≥ 6 with r < d there exists a connected graph G with monophonic radius r, monophonic diameter d and dm_t(G) = k. For positive integers a, b such that 4 ≤ a ≤ b with b ≤ 2a, there exists a connected graph G such that dm(G) = a and dm_t(G) = b. Also, if p, d and k are positive integers such that 2 ≤ d ≤ p - 2, 3 ≤ k ≤ p and p – d – k + 3 ≥ 0, there exists a connected graph G of order p, monophonic diameter d and dm_t(G) = k.