connected graphs; adjacency in a graph; regular graph; wheel
The paper studies graphs in which each pair of vertices has exactly two common neighbours. It disproves a conjectury by P. Hliněný concerning these graphs.
 Graphs '91, The Czechoslovak conference on graph theoгy held at Zemplínska Šírava in June 1991. Zbl 1170.11300
 Ore O.: Theory of Gгaphs. AMS Colloq. Publ. Providence, Rhode Island, 1962.