A241480 Numbers k such that the GCD of the x's that satisfy sigma(x) = sigma(k) is not equal to 1.

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, 68, 72, 73, 75, 80, 81, 82, 91, 98, 100, 101, 104, 106, 109, 116, 121, 122, 128, 129, 133, 134, 137, 144, 146, 148, 149, 152, 156, 157, 160, 162, 163, 169, 171, 173

Offset

1,1

Comments

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.

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

Formula

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

Prog

(PARI) sigv(n) =  select(i->sigma(i) == n, vector(n, i, i));
isok(n) = (gcd(sigv(sigma(n))) != 1);
(PARI) is(k) = gcd(invsigma(sigma(k))) > 1; \\ Amiram Eldar, Dec 19 2024, using Max Alekseyev's invphi.gp

Crossrefs

Cf. A000203, A211656, A241479.

Sequence in context: A086753 A160519 A287927 * A211656 A051204 A335154

Adjacent sequences: A241477 A241478 A241479 * A241481 A241482 A241483

Keyword

nonn

Author

Michel Marcus, Apr 23 2014

Status

approved