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A007497
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a(1) = 2, a(n) = sigma(a(n-1)).
(Formerly M0581)
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10
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2, 3, 4, 7, 8, 15, 24, 60, 168, 480, 1512, 4800, 15748, 28672, 65528, 122880, 393192, 1098240, 4124736, 15605760, 50328576, 149873152, 371226240, 1710858240, 7926750720, 33463001088, 109760857440, 384120963072
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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REFERENCES
| N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
| T. D. Noe, Table of n, a(n) for n=1..200
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FORMULA
| Conjecture : (1/2)*log(n)<a(n+1)/a(n)<2*log(n) - Benoit Cloitre (benoit7848c(AT)orange.fr), May 08 2003
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MAPLE
| A007497 := proc(n) options remember; if n <= 0 then RETURN(2) else sigma(A007497(n-1)); fi; end;
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MATHEMATICA
| a[1] = 2; a[n_] := a[n] = DivisorSigma[1, a[n-1]]; Table[a[n], {n, 30}]
NestList[ DivisorSigma[1, # ] &, 2, 27] [From Robert G. Wilson v (rgwv(AT)rgwv.com), Oct 08 2010]
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CROSSREFS
| Cf. A000203. See also the similarly defined A051572 which has a(1) = 5 instead.
Sequence in context: A120611 A092063 A126850 * A126882 A006549 A159554
Adjacent sequences: A007494 A007495 A007496 * A007498 A007499 A007500
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KEYWORD
| nonn,easy,nice
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com), Robert G. Wilson v (rgwv(AT)rgwv.com)
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EXTENSIONS
| Changed the cross-reference from the tau to the sigma-function - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Feb 17 2010
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