OFFSET
1,2
COMMENTS
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
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Feb 20 2002
STATUS
approved