This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A015841 #22 Feb 12 2021 05:33:24 %S A015841 1,2,3,6,7,8,10,17,18,24,26,42,50,52,54,56,65,75,80,81,87,90,110,111, %T A015841 114,120,126,154,164,186,216,234,244,245,306,308,338,344,350,351,371, %U A015841 372,374,416,436,456,490,528,560,570,598,626,682,689,759,777,836,866 %N A015841 Numbers k such that phi(k) | sigma(k+4). %H A015841 Ivan Neretin, <a href="/A015841/b015841.txt">Table of n, a(n) for n = 1..1000</a> %t A015841 Select[Range[3000],Divisible[DivisorSigma[1, 4 + #], EulerPhi[#]] &] (* _David Nacin_, Mar 03 2012 *) %o A015841 (PARI) isok(n) = !(sigma(n+4) % eulerphi(n)); \\ _Michel Marcus_, Feb 05 2017 %K A015841 nonn %O A015841 1,2 %A A015841 _Robert G. Wilson v_ %E A015841 a(1) = 1 inserted by _Ivan Neretin_, Feb 05 2017