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A036239
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Number of 2-element intersecting families of an n-element set; number of 2-way interactions when 2 subsets of power set on {1..n} are chosen at random.
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27
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0, 2, 15, 80, 375, 1652, 7035, 29360, 120975, 494252, 2007555, 8120840, 32753175, 131818052, 529680075, 2125927520, 8525298975, 34165897052, 136857560595, 548011897400, 2193792030375, 8780400395252, 35137296305115, 140596265198480
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| 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 intersecting but for which x is not a subset of y and y is not a subset of x, or 1) x and y are intersecting and for which either x is a proper subset of y or y is a proper subset of x. - Ross La Haye (rlahaye(AT)new.rr.com), Jan 10 2008
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REFERENCES
| W. W. Kokko, "Interactions", manuscript, 1983.
V. Jovovic, G. Kilibarda, On the number of Boolean functions in the Post classes F^{mu}_8, Diskretnaya Matematika, 11 (1999), no. 4, 127-138 (translated in Discrete Mathematics and Applications, 9, (1999), no. 6).
Ross La Haye, Binary Relations on the Power Set of an n-Element Set, Journal of Integer Sequences, Vol. 12 (2009), Article 09.2.6. [From Ross La Haye (rlahaye(AT)new.rr.com), Feb 22 2009]
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LINKS
| T. D. Noe, Table of n, a(n) for n=1..200
Thomas Wieder, The number of certain k-combinations of an n-set, Applied Mathematics Electronic Notes, vol. 8 (2008).
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FORMULA
| a(n) = 1/2 * (4^n-3^n-2^n+1).
a(n) = 3*StirlingS2(n+1,4) + 2*StirlingS2(n+1,3). - Ross La Haye (rlahaye(AT)new.rr.com), Jan 10 2008
a(n) = A006516(n) - A003462(n) [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jun 05 2009]
Contribution from Harvey P. Dale, May 11 2011. (Start)
a(0)=0, a(1)=2, a(2)=15, a(3)=80, a(n)=10*a(n-1)-35*a(n-2)+50*a(n-3)- 24*a(n-4).
G.f.: x^2*(2-5*x)/(1-10*x+35*x^2-50*x^3+24*x^4). (End)
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MATHEMATICA
| LinearRecurrence[{10, -35, 50, -24}, {0, 2, 15, 80}, 40] (* or *) With[{c=1/2!}, Table[c(4^n-3^n-2^n+1), {n, 40}]] (* From Harvey P. Dale, May 11 2011 *)
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PROG
| (Sage) [(4^n - 2^n)/2-(3^n - 1)/2 for n in xrange(1, 24)] # [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jun 05 2009]
(PARI) a(n)=(4^n-3^n-2^n+1)/2 \\ Charles R Greathouse IV, Jul 25 2011
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CROSSREFS
| Cf. A036240, A051180-A051185, A028243, A032263.
A003462, A006516, A016127, A016129, A016130, A016311, A016316, A016321, A016325 [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jun 05 2009]
Sequence in context: A102289 A041243 A056079 * A109725 A057152 A002740
Adjacent sequences: A036236 A036237 A036238 * A036240 A036241 A036242
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KEYWORD
| nonn,easy,nice
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com).
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