A067236 Numbers n such that gcd(sigma(n), sigma(sigma(n)))=1.

1, 2, 3, 4, 7, 9, 16, 18, 21, 22, 25, 31, 32, 49, 50, 52, 64, 66, 70, 81, 93, 94, 97, 98, 100, 115, 119, 121, 127, 144, 156, 162, 169, 170, 196, 210, 214, 217, 225, 241, 242, 256, 265, 282, 289, 291, 310, 322, 337, 338, 343, 345, 357, 361, 364, 381, 382, 385, 400

Offset

1,2

Comments

In other words, numbers n such that A000203(n) is in A014567. - Ivan Neretin, Feb 08 2017

Links

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

Mathematica

ds1Q[n_]:=Module[{ds=DivisorSigma[1, n]}, GCD[ds, DivisorSigma[1, ds]]==1]; Select[Range[500], ds1Q] (* Harvey P. Dale, Jun 13 2014 *)

Prog

(PARI) isok(n) = gcd(sigma(n), sigma(sigma(n))) == 1; \\ Michel Marcus, Feb 08 2017

Crossrefs

Cf. A000203, A051027.

Sequence in context: A027947 A324736 A293853 * A320357 A250255 A070763

Adjacent sequences: A067233 A067234 A067235 * A067237 A067238 A067239

Keyword

nonn

Author

Benoit Cloitre, Feb 20 2002

Status

approved