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 A087249 #18 Jun 15 2024 05:49:54 %S A087249 3,6,7,10,11,14,15,17,19,21,22,23,30,31,33,34,35,38,39,42,43,46,47,51, %T A087249 53,55,57,58,59,62,65,66,67,69,70,71,77,78,79,82,83,85,86,87,89,91,93, %U A087249 94,95,97,102,103,105,106,107,110,111,114,115,118,119,123,127,129,130 %N A087249 Squarefree numbers k such that sigma(k) is not squarefree. %H A087249 Amiram Eldar, <a href="/A087249/b087249.txt">Table of n, a(n) for n = 1..10000</a> %e A087249 For k=7: sigma(7) = 2*2*2 = 8. %e A087249 For k=10: sigma(10) = 1 + 2 + 5 + 10 = 18 = 2*3*3. %t A087249 Select[Range[150],SquareFreeQ[#]&&!SquareFreeQ[DivisorSigma[1,#]]&] (* _Harvey P. Dale_, Feb 06 2011 *) %o A087249 (PARI) is(k) = issquarefree(k) && !issquarefree(sigma(k)); \\ _Amiram Eldar_, Jun 15 2024 %Y A087249 Complement of A065301 within A005117. %Y A087249 Cf. A065303, A087248. %K A087249 nonn %O A087249 1,1 %A A087249 _Labos Elemer_, Sep 05 2003