A171258 - OEIS

%I #21 Jul 29 2024 19:01:17

%S 630,5544,11160,18810,27000,57000,80388,161820,178020,182880,242820,

%T 265608,388620,391500,447678,465192,522522,671760,690120,711000,

%U 775170,826500,901170,1051830,1102290,1157130,1418160,1578330,1679400,1812384,1874520,1993824

%N Numbers n such that sigma(n) = 13*phi(n) (where sigma=A000203, phi=A000010).

%H Amiram Eldar, <a href="/A171258/b171258.txt">Table of n, a(n) for n = 1..10000</a> (calculated using data from Jud McCranie, terms 1..1000 from Donovan Johnson)

%H Kevin A. Broughan and Daniel Delbourgo, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL16/Broughan/broughan26.html">On the Ratio of the Sum of Divisors and Euler’s Totient Function I</a>, Journal of Integer Sequences, Vol. 16 (2013), Article 13.8.8.

%H Kevin A. Broughan and Qizhi Zhou, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL17/Broughan/bro32.html">On the Ratio of the Sum of Divisors and Euler's Totient Function II</a>, Journal of Integer Sequences, Vol. 17 (2014), Article 14.9.2.

%t Select[Range[2*10^6],DivisorSigma[1,#]==13EulerPhi[#]&] (* _Harvey P. Dale_, Mar 29 2018 *)

%o (PARI) for(k=1,2e6, sigma(k) - 13*eulerphi(k) || print1(k", "));

%Y Cf. A062699, A068391, A068390, A136547, A104900, A136540, A104901, A163667, A171256, A171257, A104902, A171259, A171260, A104903.

%K nonn

%O 1,1

%A _Farideh Firoozbakht_ and _M. F. Hasler_, Mar 19 2010