A070286 Numbers k such that the equation sigma(sigma(x)) = k has solutions.

1, 4, 7, 8, 12, 14, 15, 24, 28, 32, 39, 42, 56, 60, 63, 72, 80, 84, 90, 91, 96, 104, 112, 114, 120, 124, 126, 128, 133, 160, 168, 171, 186, 195, 210, 216, 224, 234, 240, 248, 252, 255, 256, 260, 280, 288, 294, 300, 308, 312, 336, 360, 363, 372, 378, 384, 392

Offset: 1

Author

Benoit Cloitre, May 10 2002

Keywords

• easy
• nonn

Comments

Complement of A162923. - Jaroslav Krizek, Jul 19 2009

Links

• Ivan Neretin, Table of n, a(n) for n = 1..10000
• Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

Crossrefs

Cf. A051027, A162923.

Programs

Mathematica

mx = 400; Select[Union@Table[Nest[DivisorSigma[1, #] &, n, 2], {n, mx}], # < mx &] (* Ivan Neretin, Feb 12 2018 *)

PARI

is(k) = {my(v = invsigma(k)); for(i = 1, #v, if(invsigmaNum(v[i]) > 0, return(1))); 0;} \\ Amiram Eldar, Dec 26 2024, using Max Alekseyev's invphi.gp