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 A228549 #18 Apr 02 2023 09:49:36
%S A228549 1,-1,-2,3,2,-1,-5,25,-20,-16,-7,23,-39,25,309,455,901,-2168,920,1047,
%T A228549 2302,-1536,3272,161162,-289493,-529263,1418949,-1562590,-579999,
%U A228549 15145,5844471,5244416,-65749104,176617384,2077411081,7381004327,10211507191,-10333531933,7086785521,8502336136
%N A228549 Determinant of the n X n matrix with (i,j)-entry equal to 1 or 0 according as i + j is squarefree or not.
%C A228549 Conjecture: a(n) is always nonzero. Moreover, |a(n)|^(1/n) tends to infinity.
%C A228549 We have verified that a(n) is nonzero for all n <= 500.
%H A228549 Zhi-Wei Sun, <a href="/A228549/b228549.txt">Table of n, a(n) for n = 1..200</a>
%e A228549 a(1) = 1 since 1 + 1 = 2 is squarefree.
%t A228549 a[n_]:=a[n]=Det[Table[If[SquareFreeQ[i+j]==True,1,0],{i,1,n},{j,1,n}]]
%t A228549 Table[a[n],{n,1,20}]
%t A228549 Table[With[{nn=x},Det[Table[If[SquareFreeQ[i+j],1,0],{i,nn},{j,nn}]]],{x,40}] (* _Harvey P. Dale_, Apr 02 2023 *)
%Y A228549 Cf. A005117, A228548, A069191, A228552.
%K A228549 sign
%O A228549 1,3
%A A228549 _Zhi-Wei Sun_, Aug 25 2013