有向圈与有向环面的直径变化

In this paper, we study two parameters concerningwith the diameter variation under the addition or deletion of arcsin a digraph $G$: $D^{-0}(G)$ is the maximum number of arcs theaddition of which dose not change the diameter of $G$; $D^{+k}(G)$is the minimum number of arcs the deletion of which increases thediameter of $G$ by at least $k$. We give a formula for $D^{-0}(G)$if $G$ is vertex transitive and every vertex has a unique vertexwhich is farthest from it. As consequences, the values of $D^{-0}$for the directed cycle and the directed torus can be determined. Thevalues of $D^{+k}$ for these two digraphs are also determined.