OFFSET
1,2
COMMENTS
If m*k is in the sequence with m and k coprime, then m and k must be in the sequence. - Robert Israel, Mar 29 2019
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Harvey P. Dale)
FORMULA
abs(mu(sigma_2(a(n))) = 1.
MAPLE
select(n -> numtheory:-issqrfree(numtheory:-sigma[2](n)), [$1..1000]); # Robert Israel, Mar 29 2019
MATHEMATICA
Do[s=MoebiusMu[DivisorSigma[2, n]]; If[ !Equal[s, 0], Print[n]], {n, 1, 1000}]
Select[Range[200], SquareFreeQ[DivisorSigma[2, #]]&] (* Harvey P. Dale, Jun 17 2014 *)
PROG
(PARI) isok(n) = issquarefree(sigma(n, 2)); \\ Michel Marcus, Mar 29 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Mar 19 2003
STATUS
approved