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 A216042 #21 Feb 14 2021 18:33:34
%S A216042 1,3,53,106,427,3416,6806,13665,27330,130030,220227,437666
%N A216042 Numbers k such that Sum_{j=1..k-1} (2*j)!/4^j is an integer.
%C A216042 Next term > 1900000.
%t A216042 seq={}; sum=0; fak=1; k=0; While[k<10000, sum+=fak; If[Denominator[sum]==1, AppendTo[seq,k+1]]; k++; fak*=k*(2k-1)/2;]; seq
%Y A216042 Cf. A215972, A216043, A216044, A216045.
%K A216042 nonn,more
%O A216042 1,2
%A A216042 _Vaclav Kotesovec_, Aug 30 2012