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A109110
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a(n)=2a(n-1)+a(n-2)-a(n-3); a(0)=4, a(1)=9, a(2)=20.
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0
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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; internal format)
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OFFSET
| 0,1
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COMMENTS
| Kekule numbers for certain benzenoids.
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REFERENCES
| S. J. Cyvin and I. Gutman, Kekule structures in benzenoid hydrocarbons, Lecture Notes in Chemistry, No. 46, Springer, New York, 1988 (pp. 286, 288, K{S(n)})
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FORMULA
| G.f.=(4+z-2z^2)/(1-2z-z^2+z^3).
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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);
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CROSSREFS
| Sequence in context: A020708 A144686 A144685 * A108870 A111587 A161221
Adjacent sequences: A109107 A109108 A109109 * A109111 A109112 A109113
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KEYWORD
| nonn
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AUTHOR
| Emeric Deutsch (deutsch(AT)duke.poly.edu), Jun 19 2005
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