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

1,2

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).

Mathematica

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

Prog

(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

Crossrefs

Cf. A051027, A162923.

Sequence in context: A189223 A060833 A162967 * A324978 A047536 A093494

Adjacent sequences: A070283 A070284 A070285 * A070287 A070288 A070289

Keyword

easy,nonn

Author

Benoit Cloitre, May 10 2002

Status

approved