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Search: id:A006251
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| A006251 |
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Number of n-element posets which are unions of 2 chains.
(Formerly M1216)
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+0
2
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| 1, 1, 2, 4, 10, 26, 75, 225, 711, 2311, 7725, 26313, 91141, 319749, 1134234, 4060128, 14648614, 53208998, 194423568, 714130372, 2635256408, 9764995800, 36320086418, 135548135854, 507434502474, 1904982684106, 7170113287574
(list; graph; listen)
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OFFSET
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0,3
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REFERENCES
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Problem 6342, Amer. Math. Monthly, 88 (1981), 294.
R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 6.45.
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LINKS
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T. D. Noe, Table of n, a(n) for n=0..200
Index entries for sequences related to posets
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FORMULA
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G.f.: 4/(2-2*x+sqrt(1-4*x)+sqrt(1-4*x^2)).
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CROSSREFS
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Sequence in context: A049143 A089404 A006123 this_sequence A049401 A007579 A007123
Adjacent sequences: A006248 A006249 A006250 this_sequence A006252 A006253 A006254
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KEYWORD
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nonn,nice,easy
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AUTHOR
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njas
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EXTENSIONS
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More terms from James A. Sellers (sellersj(AT)math.psu.edu), Aug 21 2000
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