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 A241480 #14 Dec 19 2024 06:15:51 %S A241480 2,3,4,5,7,8,9,12,13,18,19,22,27,29,32,36,37,43,45,48,49,50,61,64,67, %T A241480 68,72,73,75,80,81,82,91,98,100,101,104,106,109,116,121,122,128,129, %U A241480 133,134,137,144,146,148,149,152,156,157,160,162,163,169,171,173 %N A241480 Numbers k such that the GCD of the x's that satisfy sigma(x) = sigma(k) is not equal to 1. %C A241480 Apart from 1, all terms of A211656 belong here since the solutions to sigma(x)=sigma(n) form a singleton and thus their GCD is n itself. %H A241480 Amiram Eldar, <a href="/A241480/b241480.txt">Table of n, a(n) for n = 1..10000</a> %H A241480 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp). %F A241480 Numbers k such that A241479(k) is not equal to 1. %o A241480 (PARI) sigv(n) = select(i->sigma(i) == n, vector(n, i, i)); %o A241480 isok(n) = (gcd(sigv(sigma(n))) != 1); %o A241480 (PARI) is(k) = gcd(invsigma(sigma(k))) > 1; \\ _Amiram Eldar_, Dec 19 2024, using _Max Alekseyev_'s invphi.gp %Y A241480 Cf. A000203, A211656, A241479. %K A241480 nonn %O A241480 1,1 %A A241480 _Michel Marcus_, Apr 23 2014