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A115920 Numbers k such that the digits of sigma(k) are a permutation of those of k, in base 10. 14
1, 69, 258, 270, 276, 609, 639, 2391, 2556, 2931, 3409, 3678, 3679, 4291, 5092, 6937, 8251, 10231, 12087, 12931, 15480, 16387, 20850, 22644, 22893, 24369, 26145, 26442, 27846, 28764, 29880, 29958, 30823, 31812, 32658, 34207, 34758 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
There is some m > 1 such that a(n) > m*n for all n > 1. This follows from the positive density of numbers k such that sigma(k)/k > 10. - Charles R Greathouse IV, Sep 07 2012
LINKS
EXAMPLE
sigma(10231) = 11032, sigma(31812) = 81312.
MATHEMATICA
Select[Range[35000], Sort[IntegerDigits[#]]==Sort[ IntegerDigits[ DivisorSigma[ 1, #]]]&] (* Harvey P. Dale, May 09 2013 *)
PROG
(Python)
from sympy import divisor_sigma
A115920_list = [n for n in range(1, 10**4) if sorted(str(divisor_sigma(n))) == sorted(str(n))] # Chai Wah Wu, Dec 13 2015
(PARI) isok(n) = vecsort(digits(n)) == vecsort(digits(sigma(n))); \\ Michel Marcus, Dec 13 2015 and May 27 2018
CROSSREFS
Sequence in context: A249899 A009434 A228671 * A183447 A296126 A188546
KEYWORD
nonn,base
AUTHOR
Giovanni Resta, Feb 06 2006
STATUS
approved

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Last modified May 21 09:41 EDT 2024. Contains 372733 sequences. (Running on oeis4.)