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A172516 Least number k such that sigma(k) >= 2^n. 1
2, 3, 6, 10, 18, 30, 60, 108, 180, 360, 720, 1260, 2520, 5040, 9240, 17640, 35280, 65520, 131040, 257040, 498960, 982800, 1884960, 3603600, 7207200, 14414400, 28274400, 56548800, 110270160, 220540320, 428828400, 845404560, 1690809120 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For n-bit arithmetic, m=a(n)-1 is the largest number for which sigma(m) can be computed without overflow. This is a subsequence of the highly abundant numbers, A002093, which is very useful for computing this sequence. a(63) is 1454751268447276800.

LINKS

T. D. Noe, Table of n, a(n) for n=1..64

FORMULA

a(n) <= 2 * a(n-1)

MATHEMATICA

k=1; Table[While[DivisorSigma[1, k]<2^n, k++ ]; k, {n, 20}]

CROSSREFS

A141847 (least number k such that sigma2(k) >= 2^n)

Sequence in context: A066067 A121364 A215006 * A102702 A181532 A077930

Adjacent sequences:  A172513 A172514 A172515 * A172517 A172518 A172519

KEYWORD

nonn

AUTHOR

T. D. Noe, Feb 05 2010

STATUS

approved

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Last modified August 8 08:27 EDT 2020. Contains 336293 sequences. (Running on oeis4.)