A015866 - OEIS

%I #37 Feb 10 2024 11:35:24

%S 20,155,182,184,203,264,621,650,702,852,893,944,1343,1357,2024,2544,

%T 2990,4130,4183,4450,5428,5835,6149,6313,6572,8177,8695,11208,11333,

%U 11991,12444,12561,12859,13595,14857,16815,18203,18330,18430,19089

%N Numbers k such that sigma(k) = sigma(k+6).

%C Up to 10^13, sigma(k-6) = sigma(k) = sigma(k+6) only for k = 33227, 604453 and 4223105512993. - _Giovanni Resta_, Mar 03 2014

%D J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 20, pp 7, Ellipses, Paris 2008.

%H Giovanni Resta, <a href="/A015866/b015866.txt">Table of n, a(n) for n = 1..10000</a> (first 1000 terms from Donovan Johnson)

%H A. Weingartner, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL14/Weingartner/wein3.html">On the Solutions of sigma(n) = sigma(n+k)</a>, Journal of Integer Sequences, Vol. 14 (2011), #11.5.5.

%t Select[Range[20000], DivisorSigma[1, #]==DivisorSigma[1, # + 6] &] (* _Vincenzo Librandi_, Mar 10 2014 *)

%o (PARI) is(n)=sigma(n)==sigma(n+6) \\ _Charles R Greathouse IV_, Mar 09 2014

%Y Cf. A002961, A007373, A015861, A015863, A015865, A015867, A015876, A015877, A015880, A015881, A015882, A015883, A181647.

%K nonn

%O 1,1

%A _Robert G. Wilson v_