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 A086368 #17 Feb 24 2025 02:03:35 %S A086368 4,8,9,16,18,20,25,36,45,49,50,64,72,80,100,104,116,117,121,128,144, %T A086368 148,169,180,196,200,208,225,234,242,244,256,261,289,292,296,320,325, %U A086368 333,361,369,404,436,441,450,452,464,488,512,529,548,549,576,578,584,592 %N A086368 Nonsquarefree numbers k such that sigma(k) is squarefree. %H A086368 Robert Israel, <a href="/A086368/b086368.txt">Table of n, a(n) for n = 1..10000</a> %e A086368 k = 45 = 3*3*5 and sigma(45) = 78 = 2*3*13. %p A086368 filter:= n -> not numtheory:-issqrfree(n) and numtheory:-issqrfree(numtheory:-sigma(n)): %p A086368 select(filter, [$1..1000]); # _Robert Israel_, May 11 2018 %t A086368 q[k_] := ! SquareFreeQ[k] && SquareFreeQ[DivisorSigma[1, k]]; Select[Range[600], q] (* _Amiram Eldar_, Feb 24 2025 *) %o A086368 (PARI) isok(k) = !issquarefree(k) && issquarefree(sigma(k)); \\ _Amiram Eldar_, Feb 24 2025 %Y A086368 Intersection of A013929 and A065300. %Y A086368 Cf. A000203 (sigma), A005117, A065303, A087249, A065301. %K A086368 nonn %O A086368 1,1 %A A086368 _Labos Elemer_, Sep 05 2003