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A038048
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a(n) = (n-1)! * sigma(n).
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24
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1, 3, 8, 42, 144, 1440, 5760, 75600, 524160, 6531840, 43545600, 1117670400, 6706022400, 149448499200, 2092278988800, 40537905408000, 376610217984000, 13871809695744000, 128047474114560000, 5109094217170944000
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OFFSET
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1,2
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COMMENTS
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sigma(n) = A000203(n) is the sum of the divisors of n.
Number of labeled regular octopi (or octopuses, cycles of ordered sets all the same size).
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REFERENCES
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F. Bergeron, G. Labelle and P. Leroux, Combinatorial Species and Tree-Like Structures, Camb. 1998, p. 56 (1.4.67).
L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 159, #10, A(n,1).
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LINKS
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FORMULA
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E.g.f.: log(f(x)), where f(x) = o.g.f. for partitions (A000041), Product_{k>=1} 1/(1 - x^k). - N. J. A. Sloane
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EXAMPLE
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a(6) = 5! * (1 + 2 + 3 + 6) = 1440 = 6! * (1 + 1/2 + 1/3 + 1/6).
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MAPLE
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a := n -> n!*add(1/j, j=numtheory:-divisors(n)): seq(a(n), n=1..23); # Emeric Deutsch, Jul 24 2005
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MATHEMATICA
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PROG
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(Sage)
A038048 = lambda n: factorial(n-1)*sigma(n, 1)
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CROSSREFS
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KEYWORD
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nonn,nice,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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