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Search: id:A084869
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| A084869 |
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Number of 2-multiantichains of an n-set. |
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+0
16
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| 1, 2, 5, 17, 71, 317, 1415, 6197, 26591, 112157, 466775, 1923077, 7863311, 31972397, 129459335, 522571157, 2104535231, 8460991037, 33972711095, 136277478437, 546270602351, 2188566048077, 8764718254055, 35090241492917
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OFFSET
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0,2
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COMMENT
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Let P(A) be the power set of an n-element set A. Then a(n) = the number of pairs of elements {x,y} of P(A) for which either 0) x and y are disjoint and for which x is not a subset of y and y is not a subset of x, or 1) x and y are intersecting but for which x is not a subset of y and y is not a subset of x, or 2) x = y. - Ross La Haye (rlahaye(AT)new.rr.com), Jan 11 2008 Ross
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LINKS
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Goran Kilibarda and Vladeta Jovovic, Antichains of Multisets, J. Integer Seqs., Vol. 7, 2004.
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FORMULA
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1/2!*(4^n - 2*3^n+3*2^n).
a(n) = 3*StirlingS2(n+1,4) + StirlingS2(n+1,3) + StirlingS2(n+1,2) + 1. - Ross La Haye (rlahaye(AT)new.rr.com), Jan 11 2008 Ross
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CROSSREFS
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Cf. A016269, A047707, A051112-A051118, A084869-A084883.
Cf. A000392, A032263, A000079.
Sequence in context: A049774 A139402 A057219 this_sequence A005966 A101900 A082282
Adjacent sequences: A084866 A084867 A084868 this_sequence A084870 A084871 A084872
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KEYWORD
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nonn
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AUTHOR
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Goran Kilibarda, Vladeta Jovovic (vladeta(AT)Eunet.yu), Jun 10 2003
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