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A049056
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Number of minimal ordered covers of a labeled n-set.
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2
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1, 1, 3, 19, 207, 3691, 103263, 4415419, 283796607, 27094905451, 3813398797023, 786844659227419, 237151202183603007, 104128385332221915211, 66478899089080159079583, 61624041121329496987905019
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OFFSET
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0,3
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REFERENCES
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R. J. Clarke, Covering a set by subsets, Discrete Math., 81 (1990), 147-152.
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LINKS
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Vincenzo Librandi, Table of n, a(n) for n = 0..100
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FORMULA
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E.g.f.: Sum((exp(x)-1)^n*exp(x*(2^n-n-1)), n=0..infinity), cf. A046165. - Vladeta Jovovic, Sep 01 2005
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MATHEMATICA
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a[0] = 1; a[n_] := Sum[ (-1)^i*Binomial[k, i]*(2^k-1-i)^n, {k, 0, n}, {i, 0, k} ]; Table[a[n], {n, 0, 15}] (* Jean-François Alcover, Jan 27 2012, after Michael Somos *)
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PROG
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(PARI) {a(n)=sum(k=0, n, sum(i=0, k, (-1)^i*binomial(k, i)*(2^k-1-i)^n))} /* Michael Somos, Oct 16 2006 */
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CROSSREFS
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Row sums of A049055.
Sequence in context: A052886 A180563 A079144 * A204262 A165356 A000275
Adjacent sequences: A049053 A049054 A049055 * A049057 A049058 A049059
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KEYWORD
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nonn,easy,nice
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AUTHOR
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N. J. A. Sloane, Michael Somos
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STATUS
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approved
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