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Search: id:A130809
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| A130809 |
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If X_1,...,X_n is a partition of a 2n-set X into 2-blocks then a(n) is equal to the number of 3-subsets of X containing none of X_i, (i=1,...n). |
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5
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| 8, 32, 80, 160, 280, 448, 672, 960, 1320, 1760, 2288, 2912, 3640, 4480, 5440, 6528, 7752, 9120, 10640, 12320, 14168, 16192, 18400, 20800, 23400, 26208, 29232, 32480, 35960, 39680, 43648, 47872, 52360, 57120, 62160, 67488, 73112, 79040, 85280
(list; graph; listen)
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OFFSET
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3,1
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COMMENT
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Number of n permutations (n>=3)of 3 objects u,v,z, with repetition allowed, containing n-3 u's. Example: if n=3 then n-3 =zero u, a(1)=8 because we have vvv, vvz, vzv, zvv, zzv, zvz, zzv, zzz, A038207 formatted as a triangular array: diagonal: 8, 32, 80, 160, 280, 448, 672... [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Aug 05 2008]
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LINKS
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Milan Janjic, Two Enumerative Functions
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FORMULA
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a(n)=4/3*n*(n-1)*(n-2)
a(n)=C(n,n-3)*2^3,n>=3. - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Dec 07 2007
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MAPLE
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a:=n->4/3*n*(n-1)*(n-2);
seq(binomial(n, n-3)*2^3, n=3..41); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Dec 07 2007
(Maple) seq(binomial(n+2, 3)*2^3, n=1..22); [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Aug 05 2008]
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CROSSREFS
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Sequence in context: A009245 A018842 A139098 this_sequence A018839 A008412 A014819
Adjacent sequences: A130806 A130807 A130808 this_sequence A130810 A130811 A130812
A038207, A000079, A001787, A001788, A001789, A003472, A054849, A002409, A054851, A140325, A140354, A046092 [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Aug 05 2008]
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KEYWORD
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nonn,new
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AUTHOR
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Milan R. Janjic (agnus(AT)blic.net), Jul 16 2007
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