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%I #3 Mar 30 2012 16:50:35
%S 0,1,3,6,10,15,21,28,35
%N Maximal number of edges in a doubly linear graph with n nodes.
%C a(9) is known to be 34 or 35. I have put 35 here so as to distinguish this sequence from the triangular numbers. But the true value is (or was in 1997 at least) not known.
%D L. W. Beineke, Biplanar graphs: a survey, Computers Math. Applic., 34 (1997), 1-8.
%D Hutchinson, Joan P.; Shermer, Thomas; and Vince, Andrew; On representations of some thickness-two graphs. Comput. Geom. 13 (1999), 161-171.
%F For n >= 10, 6n-20 <= a(n) <= 6n-18.
%K nonn,more
%O 1,3
%A _N. J. A. Sloane_, Dec 02 2006