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%I #13 Mar 29 2019 21:06:22
%S 1,2,3,4,5,8,9,10,11,12,13,16,17,18,19,20,22,23,25,29,31,32,37,38,40,
%T 44,47,48,49,50,52,53,58,59,61,62,64,67,68,71,72,73,75,76,79,80,83,88,
%U 89,92,97,98,99,101,103,109,113,116,117,118,121,122,124,127,128,131,137
%N A001157(n) is squarefree: sum of squares of divisors of n is squarefree.
%C If m*n is in the sequence with m and n coprime, then m and n must be in the sequence. - _Robert Israel_, Mar 29 2019
%H Harvey P. Dale, <a href="/A080401/b080401.txt">Table of n, a(n) for n = 1..1000</a>
%F Abs[mu[sigma[2, a(n)]]]=1.
%p select(n -> numtheory:-issqrfree(numtheory:-sigma[2](n)), [$1..1000]); # _Robert Israel_, Mar 29 2019
%t Do[s=MoebiusMu[DivisorSigma[2, n]]; If[ !Equal[s, 0], Print[n]], {n, 1, 1000}]
%t Select[Range[200],SquareFreeQ[DivisorSigma[2,#]]&] (* _Harvey P. Dale_, Jun 17 2014 *)
%o (PARI) isok(n) = issquarefree(sigma(n, 2)); \\ _Michel Marcus_, Mar 29 2019
%Y Cf. A001157, A005117, A065300.
%K nonn
%O 1,2
%A _Labos Elemer_, Mar 19 2003
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