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Search: id:A005580
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| A005580 |
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Least number of distinct prime factors in odd numbers having an abundancy index > n.
(Formerly M2740)
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+0
2
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| 3, 8, 21, 54, 141, 372, 995, 2697, 7397, 20502, 57347, 161658
(list; graph; listen)
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OFFSET
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2,1
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COMMENT
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The abundancy index of a number k is sigma(k)/k. - T. D. Noe (noe(AT)sspectra.com), May 08 2006
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
Laatsch, Richard; Measuring the abundancy of integers. Math. Mag. 59 (1986), no. 2, 84-92.
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FORMULA
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a(n)=A005579(2n)-1 - T. D. Noe (noe(AT)sspectra.com), May 08 2006
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MATHEMATICA
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prod=1; k=1; Table[While[prod<=n, k++; prod=prod*Prime[k]/(Prime[k]-1)]; k, {n, 2, 12}] - T. D. Noe (noe(AT)sspectra.com), May 08 2006
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CROSSREFS
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Cf. A005579 (least number of distinct prime factors in even numbers having an abundancy index > n).
Sequence in context: A127358 A077849 A135473 this_sequence A027932 A084625 A001906
Adjacent sequences: A005577 A005578 A005579 this_sequence A005581 A005582 A005583
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KEYWORD
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nonn
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com).
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EXTENSIONS
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Edited by T. D. Noe (noe(AT)sspectra.com), May 08 2006
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