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 A173186 #11 Feb 25 2021 08:30:54
%S A173186 1,3,5,7,8,9,10,11,13,15,17,19,21,23,24,25,26,27,28,29,31,33,35,37,39,
%T A173186 41,43,44,45,46,47,48,49,50,51,53,55,57,59,61,62,63,64,65,67,69,71,73,
%U A173186 74,75,76,77,79,80,81,82,83,85,87,89,91,93,95,97,98,99,100,101,103,105
%N A173186 Numbers k such that k^2-1 is not squarefree.
%C A173186 Complement of A067874 - _R. J. Mathar_, Feb 13 2010
%C A173186 The asymptotic density of this sequence is 1 - Product_{p prime} (1 - 2/p^2) = 0.677365... (1 - A065474). - _Amiram Eldar_, Feb 25 2021
%H A173186 Amiram Eldar, <a href="/A173186/b173186.txt">Table of n, a(n) for n = 1..10000</a>
%e A173186 5 is a term since 5^2-1 = 24 = 2^3*3 is not squarefree.
%t A173186 f[n_]:=SquareFreeQ[n]; lst={};Do[If[f[n^2-1],AppendTo[lst,n]],{n,6!}]; lst
%t A173186 Select[Range[120],!SquareFreeQ[#^2-1]&] (* _Harvey P. Dale_, Jul 16 2016 *)
%Y A173186 Cf. A013929, A065474, A067874.
%K A173186 nonn
%O A173186 1,2
%A A173186 _Vladimir Joseph Stephan Orlovsky_, Feb 12 2010
%E A173186 Definition edited by _R. J. Mathar_, Feb 13 2010