[1] HELLWIG A, VOLKMANN L. Maximally edge-connected and vertex-connected graphs and digraphs: A survey[J]. Discrete Mathematics, 2008, 308: 3265-3296. [2] GELLER D, HARARY F. Connectivity in digraphs[C]// Recent Trends in Graph Theory. Berlin/ Heidelberg: Springer, 1971, 186: 105-115. [3] CHARTRAND G, HARARY F. Graphs with prescribed connectivities[C]// Theory of Graphs. London/ New York/ San Francisco: Academic Press, 1968: 61-63. [4] BALBUENA C, CARMONA A. On the connectivity and superconnectivity of bipartite digraphs and graphs[J]. Ars Combinatoria, 2001, 61: 3-21. [5] FBREGA J, FIOL M A. Maximally connected digraphs[J]. Journal of Graph Theory, 1989, 13: 657-668. [6] HELLWIG A, VOLKMANN L. Lower bounds on the vertex-connectivity of digraphs and graphs[J]. Information Processing Letters, 2006, 99: 41-46.