Extremal graphs of diameter two and given maximum degree, embeddable in a fixed surface

It is known that for each d there exists a graph of diameter two and maximum degree d which has at least ⌈(d/2)⌉ ⌈(d + 2)/2⌉ vertices. In contrast with this, we prove that for every surface S there is a constant ds such that each graph of diameter two and maximum degree d ≥ ds, which is embeddable in S, has at most ⌊(3/2)d⌋ + 1 vertices. Moreover, this upper bound is best possible, and we show that extremal graphs can be found among surface triangulations. © 1997 John Wiley & Sons, Inc.