A162923 Numbers k such that the equation sigma(sigma(x)) = k has no solutions.

2, 3, 5, 6, 9, 10, 11, 13, 16, 17, 18, 19, 20, 21, 22, 23, 25, 26, 27, 29, 30, 31, 33, 34, 35, 36, 37, 38, 40, 41, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 57, 58, 59, 61, 62, 64, 65, 66, 67, 68, 69, 70, 71, 73, 74, 75, 76, 77, 78, 79, 81, 82, 83, 85, 86, 87, 88, 89, 92, 93

Offset

1,1

Links

Ivan Neretin, Table of n, a(n) for n = 1..10000

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

Formula

{A000027} minus {A070286}.

Mathematica

mx = 100; Complement[Range[mx], Table[Nest[DivisorSigma[1, #] &, n, 2], {n, mx}]] (* Ivan Neretin, Feb 12 2018 *)

Prog

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

Crossrefs

Complement of A070286.

Cf. A051027.

Sequence in context: A069861 A379774 A047449 * A107040 A045989 A129832

Adjacent sequences: A162920 A162921 A162922 * A162924 A162925 A162926

Keyword

nonn

Author

Jaroslav Krizek, Jul 17 2009

Status

approved