[1] F. Buckley, Z. Miller, P. J. Slater: On graphs containing a given graph as center. J. Graph Theory 5 (1981), 427–434. MR 0635706

[2] G. Chartrand, D. Erwin, M. Raines, P. Zhang: Strong distance in strong digraphs. J. Combin. Math. Combin. Comput. 31 (1999), 33–44. MR 1726945

[3] G. Chartrand, D. Erwin, M. Raines, P. Zhang: On strong distance in strong oriented graphs. Congr. Numer. 141 (1999), 49–63. MR 1744211

[4] G. Chartrand, L. Lesniak: Graphs $\&$ Digraphs, third edition. Chapman $\&$ Hall, New York, 1996. MR 1408678

[5] G. Chartrand, O. R. Oellermann, S. Tian, H. B. Zou: Steiner distance in graphs. Čas. Pěst. Mat. 114 (1989), 399–410. MR 1027236