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A067236 Numbers n such that gcd(sigma(n), sigma(sigma(n)))=1. 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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified June 16 14:56 EDT 2019. Contains 324152 sequences. (Running on oeis4.)