A115920 - OEIS

A115920

Numbers k such that the digits of sigma(k) are a permutation of those of k, in base 10.

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

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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

V. Raman, Table of n, a(n) for n = 1..10000

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