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 A216149 #22 Feb 14 2021 18:34:24
%S A216149 1,3,16
%N A216149 Numbers k such that Sum_{j=1..k-1} (3*j)!/5^j is an integer.
%C A216149 The next term, if it exists, is greater than 1000000. - _Vaclav Kotesovec_, Sep 11 2012
%t A216149 seq={}; sum=0; fak=1; k=0; While[k<10000, sum+=fak; If[Denominator[sum]==1, AppendTo[seq,k+1]]; k++; fak*=3*k*(3*k-1)*(3*k-2)/5;]; seq
%t A216149 Select[Range[20],IntegerQ[Sum[(3k)!/5^k,{k,#-1}]]&] (* _Harvey P. Dale_, Apr 07 2019 *)
%Y A216149 Cf. A215972, A216042, A216043, A216044, A216045.
%K A216149 nonn,more,bref
%O A216149 1,2
%A A216149 _Vaclav Kotesovec_, Sep 02 2012