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 A005580 Least number of distinct prime factors in odd numbers having an abundancy index > n. (Formerly M2740) 1
 3, 8, 21, 54, 141, 372, 995, 2697, 7397, 20502, 57347, 161658 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,1 COMMENTS The abundancy index of a number k is sigma(k)/k. - T. D. Noe, May 08 2006 REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS R. K. Guy, Letter to N. J. A. Sloane, 1988-04-12 (annotated scanned copy) Richard Laatsch, Measuring the abundancy of integers, Mathematics Magazine 59 (2) (1986) 84-92. FORMULA a(n) = A005579(2n)-1. - T. D. Noe, May 08 2006 MATHEMATICA prod=1; k=1; Table[While[prod<=n, k++; prod=prod*Prime[k]/(Prime[k]-1)]; k, {n, 2, 12}] (* T. D. Noe, May 08 2006 *) CROSSREFS Cf. A005579 (least number of distinct prime factors in even numbers having an abundancy index > n). Sequence in context: A135473 A242452 A190139 * A231222 A231436 A027932 Adjacent sequences:  A005577 A005578 A005579 * A005581 A005582 A005583 KEYWORD nonn,more AUTHOR EXTENSIONS Edited by T. D. Noe, May 08 2006 STATUS approved

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