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A089433
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Number of noncrossing connected graphs on n nodes having exactly two interior faces.
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0
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2, 30, 315, 2856, 23940, 191268, 1480050, 11196900, 83304936, 611931320, 4450217772, 32104210320, 230080173960, 1639890119016, 11634355574100, 82216112723640, 579022013389050, 4065827626164150, 28475852003986695
(list; graph; refs; listen; history; internal format)
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OFFSET
| 4,1
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REFERENCES
| P. Flajolet and M. Noy, Analytic combinatorics of non-crossing configurations, Discrete Math. 204 (1999), 203-229.
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FORMULA
| a(n)=n*binomial(3n-3, n-4)/2.
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EXAMPLE
| a(4)=2 because the only connected graphs on the nodes A,B,C,D having exactly two interior faces are {AB,BC,CD,DA,AC} and {AB,BC,CD,DA,BD}.
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CROSSREFS
| Cf. A007297.
Sequence in context: A189103 A036351 A189770 * A152277 A083446 A091345
Adjacent sequences: A089430 A089431 A089432 * A089434 A089435 A089436
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KEYWORD
| nonn
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AUTHOR
| Emeric Deutsch (deutsch(AT)duke.poly.edu), Dec 28 2003
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