The vertex detour hull number of a graph

Santhakumaran, A. ; Ullas Chandran, S.

Discussiones Mathematicae. Graph Theory, Tome 32 (2012) no. 2, pp. 321-330.

Résumé

For vertices x and y in a connected graph G, the detour distance D(x,y) is the length of a longest x - y path in G. An x - y path of length D(x,y) is an x - y detour. The closed detour interval I_D[x,y] consists of x,y, and all vertices lying on some x -y detour of G; while for S ⊆ V(G), I_D[S] = ⋃_x,y ∈ S I_D[x,y]. A set S of vertices is a detour convex set if I_D[S] = S. The detour convex hull [S]_D is the smallest detour convex set containing S. The detour hull number dh(G) is the minimum cardinality among subsets S of V(G) with [S]_D = V(G). Let x be any vertex in a connected graph G. For a vertex y in G, denoted by I_D[y]^x, the set of all vertices distinct from x that lie on some x - y detour of G; while for S ⊆ V(G), I_D[S]^x = ⋃_y ∈ S I_D[y]^x. For x ∉ S, S is an x-detour convex set if I_D[S]^x = S. The x-detour convex hull of S, [S]^x_D is the smallest x-detour convex set containing S. A set S is an x-detour hull set if [S]^x_D = V(G) -x and the minimum cardinality of x-detour hull sets is the x-detour hull number dhₓ(G) of G. For x ∉ S, S is an x-detour set of G if I_D[S]^x = V(G) - x and the minimum cardinality of x-detour sets is the x-detour number dₓ(G) of G. Certain general properties of the x-detour hull number of a graph are studied. It is shown that for each pair of positive integers a,b with 2 ≤ a ≤ b+1, there exist a connected graph G and a vertex x such that dh(G) = a and dhₓ(G) = b. It is proved that every two integers a and b with 1 ≤ a ≤ b, are realizable as the x-detour hull number and the x-detour number respectively. Also, it is shown that for integers a,b and n with 1 ≤ a ≤ n -b and b ≥ 3, there exist a connected graph G of order n and a vertex x such that dhₓ(G) = a and the detour eccentricity of x, e_D(x) = b. We determine bounds for dhₓ(G) and characterize graphs G which realize these bounds.

Détail

Keywords: detour, detour number, detour hull number, x-detour number, x-detour hull number

Bibliographie

[1] F. Buckley and F. Harary, Distance in Graphs (Addison-Wesley, Reading MA, 1990).

[2] G. Chartrand, H. Escuadro and P. Zhang, Detour distance in graphs, J. Combin. Math. Combin. Comput. 53 (2005) 75-94.

[3] G. Chartrand, G.L. Johns and P. Zhang, Detour number of a graph, Util. Math. 64 (2003) 97-113.

[4] G. Chartrand, G.L. Johns and P. Zhang, On the detour number and geodetic number of a graph, Ars Combin. 72 (2004) 3-15.

[5] G. Chartrand, L. Nebesky and P. Zhang, A survey of Hamilton colorings of graphs, preprint.

[6] G. Chartrand and P. Zhang, Introduction to Graph Theory (Tata McGraw- Hill Edition, New Delhi, 2006).

[7] W. Hale, Frequency Assignment, in: Theory and Applications, Proc. IEEE 68 (1980) 1497-1514, doi: 10.1109/PROC.1980.11899.

[8] A.P. Santhakumaran and S. Athisayanathan, Connected detour number of a graph, J. Combin. Math. Combin. Comput. 69 (2009) 205-218.

[9] A.P. Santhakumaran and P. Titus, The vertex detour number of a graph, AKCE J. Graphs. Combin. 4 (2007) 99-112.

[10] A.P. Santhakumaran and S.V. Ullas Chandran, The detour hull number of a graph, communicated.