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Keywords:
the degree-diameter problem; voltage assignment and lift; dipole
Summary:
For any $d\ge 11$ we construct graphs of degree $d$, diameter $2$, and order $\frac{8}{25}d^2+O(d)$, obtained as lifts of dipoles with voltages in cyclic groups. For Cayley Abelian graphs of diameter two a slightly better result of $\frac{9}{25}d^2 + O(d)$ has been known [3] but it applies only to special values of degrees $d$ depending on prime powers.
References:
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[7] Šiagiová, J.: A Moore-like bound for graphs of diameter 2 and given degree obtained as Abelian lifts of dipoles. Acta Math. Univ. Comen. 71 (2002), 2, 157–161. MR 1980377 | Zbl 1046.05023
[8] Šiagiová, J.: A note on the McKay-Miller-Širáň graphs. J. Combinat. Theory Ser. B 81 (2001), 205–208. DOI 10.1006/jctb.2000.2006 | MR 1814904 | Zbl 1024.05039
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