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Search: id:A134169
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| A134169 |
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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 and y are intersecting and for which either x is a proper subset of y or y is a proper subset of x, or 3) x = y. |
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+0
1
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| 1, 2, 7, 29, 121, 497, 2017, 8129, 32641, 130817, 523777, 2096129, 8386561, 33550337, 134209537, 536854529, 2147450881, 8589869057, 34359607297, 137438691329, 549755289601
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OFFSET
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0,2
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FORMULA
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a(n) = 2^(n-1)*(2^n - 1) + 1 = StirlingS2(2^n,2^n - 1) + 1 = C(2^n,2) + 1 = A006516(n) + 1.
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EXAMPLE
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a(2) = 7 because for P(A) = {{},{1},{2},{1,2}} we have for case 0
{{1},{2}}, and we have for case 2 {{1},{1,2}}, {{2},{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 1.
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CROSSREFS
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Cf. A000392, A032263, A028243, A000079, A006516.
Sequence in context: A083016 A062064 A120757 this_sequence A052961 A150662 A126568
Adjacent sequences: A134166 A134167 A134168 this_sequence A134170 A134171 A134172
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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 Ross
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