This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A080401 #24 Jan 11 2025 19:56:19
%S A080401 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 A080401 44,47,48,49,50,52,53,58,59,61,62,64,67,68,71,72,73,75,76,79,80,83,88,
%U A080401 89,92,97,98,99,101,103,109,113,116,117,118,121,122,124,127,128,131,137
%N A080401 Numbers k such that the sum of the squares of the divisors of k (A001157(k)) is squarefree.
%C A080401 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
%H A080401 Amiram Eldar, <a href="/A080401/b080401.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Harvey P. Dale)
%F A080401 abs(mu(sigma_2(a(n)))) = 1.
%p A080401 select(n -> numtheory:-issqrfree(numtheory:-sigma[2](n)), [$1..1000]); # _Robert Israel_, Mar 29 2019
%t A080401 Do[s=MoebiusMu[DivisorSigma[2, n]]; If[ !Equal[s, 0], Print[n]], {n, 1, 1000}]
%t A080401 Select[Range[200],SquareFreeQ[DivisorSigma[2,#]]&] (* _Harvey P. Dale_, Jun 17 2014 *)
%o A080401 (PARI) isok(n) = issquarefree(sigma(n, 2)); \\ _Michel Marcus_, Mar 29 2019
%Y A080401 Cf. A001157, A005117, A065300, A080402 (complement).
%K A080401 nonn
%O A080401 1,2
%A A080401 _Labos Elemer_, Mar 19 2003