login
This site is supported by donations to The OEIS Foundation.

 

Logo

Many excellent designs for a new banner were submitted. We will use the best of them in rotation.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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

Laatsch, Richard; Measuring the abundancy of integers. Math. Mag. 59 (1986), no. 2, 84-92.

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.

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: A077849 A135473 A190139 * A231222 A231436 A027932

Adjacent sequences:  A005577 A005578 A005579 * A005581 A005582 A005583

KEYWORD

nonn

AUTHOR

N. J. A. Sloane.

EXTENSIONS

Edited by T. D. Noe, May 08 2006

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

Content is available under The OEIS End-User License Agreement .

Last modified April 19 17:29 EDT 2014. Contains 240767 sequences.