

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
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OFFSET

2,1


COMMENTS

The abundancy index of a number k is sigma(k)/k.  T. D. Noe, May 08 2006


REFERENCES

Laatsch, Richard; Measuring the abundancy of integers. Math. Mag. 59 (1986), no. 2, 8492.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS

Table of n, a(n) for n=2..13.
R. K. Guy, Letter to N. J. A. Sloane, 19880412 (annotated scanned copy)


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

N. J. A. Sloane.


EXTENSIONS

Edited by T. D. Noe, May 08 2006


STATUS

approved



