Maximum degree in graphs of diameter 2

It is well known that there are at most four Moore graphs of diameter 2, i.e., graphs of diameter 2, maximum degree d, and d2 + 1 vertices. The purpose of this paper is to prove that with the exception of C4, there are no graphs of diameter 2, of maximum degree d, and with d2 vertices. Copyright © 1980 Wiley Periodicals, Inc., A Wiley Company