A241480 - OEIS

%I #14 Dec 19 2024 06:15:51

%S 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 68,72,73,75,80,81,82,91,98,100,101,104,106,109,116,121,122,128,129,

%U 133,134,137,144,146,148,149,152,156,157,160,162,163,169,171,173

%N Numbers k such that the GCD of the x's that satisfy sigma(x) = sigma(k) is not equal to 1.

%C 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 Amiram Eldar, <a href="/A241480/b241480.txt">Table of n, a(n) for n = 1..10000</a>

%H Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).

%F Numbers k such that A241479(k) is not equal to 1.

%o (PARI) sigv(n) =  select(i->sigma(i) == n, vector(n, i, i));

%o isok(n) = (gcd(sigv(sigma(n))) != 1);

%o (PARI) is(k) = gcd(invsigma(sigma(k))) > 1; \\ _Amiram Eldar_, Dec 19 2024, using _Max Alekseyev_'s invphi.gp

%Y Cf. A000203, A211656, A241479.

%K nonn

%O 1,1

%A _Michel Marcus_, Apr 23 2014