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 A188049 #20 Feb 22 2021 04:03:08 %S A188049 17,18,19,40,49,51,53,55,69,82,89,91,97,99,117,118 %N A188049 Numbers k such that k^k-1 and k^k+1 are both not squarefree. %e A188049 17 is a term since 17^17-1 = 2^4*10949*1749233*2699538733 and 17^17+1 = 2*3^2*45957792327018709121. %t A188049 Select[Range@41,!(SquareFreeQ[#^#-1]||SquareFreeQ[#^#+1])&] %o A188049 (PARI) isok(k) = ! issquarefree(k^k-1) && ! issquarefree(k^k+1); \\ _Michel Marcus_, Feb 22 2021 %Y A188049 Cf. A005117, A013929, A184966, A184967, A188047. %K A188049 nonn,hard,more %O A188049 1,1 %A A188049 _Vladimir Joseph Stephan Orlovsky_, Mar 19 2011 %E A188049 a(5)-a(14) from _D. S. McNeil_, Mar 22 2011 %E A188049 a(15)-a(16) from _Amiram Eldar_, Feb 22 2021