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 A109110 a(n) = 2a(n-1) + a(n-2) - a(n-3); a(0)=4, a(1)=9, a(2)=20. 0
 4, 9, 20, 45, 101, 227, 510, 1146, 2575, 5786, 13001, 29213, 65641, 147494, 331416, 744685, 1673292, 3759853, 8448313, 18983187, 42654834, 95844542, 215360731, 483911170, 1087338529, 2443227497, 5489882353, 12335653674, 27717962204 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS KekulĂ© numbers for certain benzenoids. REFERENCES S. J. Cyvin and I. Gutman, KekulĂ© structures in benzenoid hydrocarbons, Lecture Notes in Chemistry, No. 46, Springer, New York, 1988 (pp. 286, 288, K{S(n)}) LINKS Index entries for linear recurrences with constant coefficients, signature (2,1,-1) FORMULA G.f.: (4 + z - 2z^2)/(1 - 2z - z^2 + z^3). a(n) = A052534(n+2). - R. J. Mathar, Feb 03 2014 MAPLE a[0]:=4:a[1]:=9:a[2]:=20: for n from 3 to 32 do a[n]:=2*a[n-1]+a[n-2]-a[n-3] od: seq(a[n], n=0..32); CROSSREFS Sequence in context: A020708 A144686 A144685 * A108870 A331942 A111587 Adjacent sequences:  A109107 A109108 A109109 * A109111 A109112 A109113 KEYWORD nonn,easy AUTHOR Emeric Deutsch, Jun 19 2005 STATUS approved

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Last modified May 31 00:29 EDT 2020. Contains 334747 sequences. (Running on oeis4.)