A241479 GCD of the solutions x of sigma(x) = sigma(n), where sigma(n) = A000203(n) = sum of divisors of n.

1, 2, 3, 4, 5, 1, 7, 8, 9, 1, 1, 12, 13, 1, 1, 1, 1, 18, 19, 1, 1, 22, 1, 1, 1, 1, 27, 1, 29, 1, 1, 32, 1, 1, 1, 36, 37, 1, 1, 1, 1, 1, 43, 1, 45, 1, 1, 3, 49, 50, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 61, 1, 1, 64, 1, 1, 67, 2, 1, 1, 1, 72, 73, 1, 3, 1, 1, 1, 1, 2

Offset: 1

Author

Michel Marcus, Apr 23 2014

Keywords

• nonn

Comments

A variant of A240667 without zeros.

Examples

			a(6) = 1 since sigma(6) = 12 and sigma(11) = 12 and gcd(6, 11) = 1.
		

Links

• Antti Karttunen, Table of n, a(n) for n = 1..20000
• Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

Crossrefs

Cf. A000203, A240667.

Programs

PARI

sigv(n) =  select(i->sigma(i) == n, vector(n, i, i));
a(n) = gcd(sigv(sigma(n)));

PARI

a(n) = gcd(invsigma(sigma(n))); \\ Amiram Eldar, Dec 19 2024, using Max Alekseyev's invphi.gp

Formula

a(n) = A240667(A000203(n)).