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Jun 18, 2015 at 8:54 history edited Gordon Royle CC BY-SA 3.0
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Jun 16, 2015 at 14:20 comment added user71114 I have one happy student now. Another aspect of Pappus being special is that for n=16 the max is smaller than for n=18, and for n=20 it stays the same (if I can now trust his program); whereas in general the max seems to decrease (or stay the same) as n increases. Of course these small values don't mean much.
Jun 16, 2015 at 14:11 vote accept CommunityBot
Jun 16, 2015 at 12:19 comment added Sebi Cioaba Nozaki studied a similar problem (among all k-regular graphs of given order v, find the one(s) with the smallest second eigenvalue of the adjacency matrix): arxiv.org/abs/1407.4562 Nozaki obtained a linear programming bound for graphs. With Nozaki, Koolen and Vermette, I studied a related, but different problem (given $k$ and $\lambda$, find the largest order of a $k$-regular graph with second eigenvalue at most $\lambda$); we used Nozaki's LP bound and interlacing. The Pappus graph appeared as extremal (largest cubic graph with $\lambda_2\leq \sqrt{3}$): arxiv.org/abs/1503.06286
Jun 16, 2015 at 8:45 history answered Gordon Royle CC BY-SA 3.0