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Search: id:A134165
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| A134165 |
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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 either x is a subset of y or y is a subset of x, or 1) x and y are disjoint and for which x is not a subset of y and y is not a subset of x, or 2) x and y are intersecting but for which x is not a subset of y and y is not a subset of x, or 3) x = y. |
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1
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| 1, 3, 8, 24, 86, 348, 1478, 6324, 26846, 112668, 467798, 1925124, 7867406, 31980588, 129475718, 522603924, 2104600766, 8461122108, 33972973238, 136278002724, 546271650926
(list; graph; listen)
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OFFSET
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0,2
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FORMULA
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a(n) = (1/2)(4^n - 2*3^n + 5*2^n - 2) = 3*StirlingS2(n+1,4) + StirlingS2(n+1,3) + 2*StirlingS2(n+1,2) + 1.
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EXAMPLE
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a(2) = 8 because for P(A) = {{},{1},{2},{1,2}} we have for case 0
{{},{1}}, {{},{2}}, {{},{1,2}}, and we have for case 1 {{1},{2}}, and we
have for case 3 {{},{}}, {{1},{1}}, {{2},{2}}, {{1,2},{1,2}}. There are 0
{x,y} of P(A) in this example that fall under case 2.
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CROSSREFS
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Cf. A000225, A000392, A032263, A000079.
Sequence in context: A088966 A148787 A125655 this_sequence A071016 A002104 A102919
Adjacent sequences: A134162 A134163 A134164 this_sequence A134166 A134167 A134168
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KEYWORD
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nonn
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AUTHOR
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Ross La Haye (rlahaye(AT)new.rr.com), Jan 12 2008
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