P. Kristiansen, S.M. Hedetniemi, and S. T. Hedetniemi, in Alliances in graphs, J. Combin. Math. Combin. Comput. 48 (2004), 157–177, and T. W. Haynes, S. T. Hedetniemi, and M. A. Henning, in Global defensive alliances in graphs, Electron. J. Combin. 10 (2003), introduced the defensive alliance number γa(G), strong defensive alliance number aˆ(G), and global defensive alliance number γa(G). In this paper, we consider relationships between these parameters and the domination number γ(G). For any positive integers a, b, and c satisfying a ≤ c and b ≤ c, there is a graph G with a = a(G), b = γ(G), and c = γa(G). For any positive integers a, b, and c, provided a ≤ b ≤ c and c is not too much larger than a and b, there is a graph G with γ(G) = a, γa(G) = b, and γaˆ(G) = c. Given two connected graphs H1 and H2, where order(H1) ≤ order(H2), there exists a graph G with a unique minimum defensive alliance isomorphic to H1 and a unique minimum strong defensive alliance isomorphic to H2.
The eccentricity of a vertex v of a connected graph G is the distance from v to a vertex farthest from v in G. The center of G is the subgraph of G induced by the vertices having minimum eccentricity. For a vertex v in a 2-edge-connected graph G, the edge-deleted eccentricity of v is defined to be the maximum eccentricity of v in G − e over all edges e of G. The edge-deleted center of G is the subgraph induced by those vertices of G having minimum edge-deleted eccentricity. The edge-deleted central appendage number of a graph G is the minimum difference |V (H)| − |V (G)| over all graphs H where the edgedeleted center of H is isomorphic to G. In this paper, we determine the edge-deleted central appendage number of all trees.
The directed distance $d(u,v)$ from $u$ to $v$ in a strong digraph $D$ is the length of a shortest $u-v$ path in $D$. The eccentricity $e(v)$ of a vertex $v$ in $D$ is the directed distance from $v$ to a vertex furthest from $v$ in $D$. The center and periphery of a strong digraph are two well known subdigraphs induced by those vertices of minimum and maximum eccentricities, respectively. We introduce the interior and annulus of a digraph which are two induced subdigraphs involving the remaining vertices. Several results concerning the interior and annulus of a digraph are presented.