OFFSET
1,3
COMMENTS
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.
REFERENCES
L. W. Beineke, Biplanar graphs: a survey, Computers Math. Applic., 34 (1997), 1-8.
Hutchinson, Joan P.; Shermer, Thomas; and Vince, Andrew; On representations of some thickness-two graphs. Comput. Geom. 13 (1999), 161-171.
FORMULA
For n >= 10, 6n-20 <= a(n) <= 6n-18.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
N. J. A. Sloane, Dec 02 2006
STATUS
approved