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Search: id:A056120
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| A056120 |
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a(n)=(3^3)*4^(n-3); a(0)=1, a(1)=1. |
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+0
2
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| 1, 1, 7, 27, 108, 432, 1728, 6912, 27648, 110592, 442368, 1769472, 7077888, 28311552, 113246208, 452984832, 1811939328, 7247757312, 28991029248, 115964116992, 463856467968, 185542587182
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OFFSET
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0,3
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COMMENT
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For n>=3, a(n) is equal to the number of functions f:{1,2,...,n}->{1,2,3,4} such that for fixed, different x_1, x_2, x_3 in {1,2,...,n} and fixed y_1, y_2, y_3 in {1,2,3,4} we have f(x_i)<>y_i, (i=1,2,...,n). - Milan R. Janjic (agnus(AT)blic.net), May 13 2007
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LINKS
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Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets
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FORMULA
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a(n)=4a(n-1)+[(-1)^n]*C(3, 3-n). G.f.(x)=(1-x)^3/(1-4x).
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CROSSREFS
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Cf. A055841.
First differences of A002063.
Sequence in context: A054485 A090856 A055917 this_sequence A048711 A118101 A147996
Adjacent sequences: A056117 A056118 A056119 this_sequence A056121 A056122 A056123
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KEYWORD
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easy,nonn
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AUTHOR
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Barry E. Williams, Jul 05 2000
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