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 A280723 #37 Jan 15 2017 13:21:23
%S A280723 2,16,384,6144,819200,19660800,7707033600,3288334336,14205604331520,
%T A280723 568224173260800,3741775508275200,179605224397209600,
%U A280723 135982707495615332352,1410191040695270113280,169222924883432413593600,10830267192539674469990400,1655509272671188586751590400
%N A280723 a(n) is the denominator of 6 * Sum_{k=0..n} ((k+1)/(n-k+1)^2) * (Catalan(k)/(2^(2*k+1)))^2.
%C A280723 The series A281070(n)/a(n) is absolutely convergent to Pi.
%t A280723 a[n_]=6(Sum[(1/(n-k+1)^2)((CatalanNumber[k])/(2^(2k+1)))^2(k+1), {k, 0, n}]); Denominator /@a/@ Range[0, 10]
%Y A280723 Cf. A000108 (Catalan), A281070 (numerators).
%K A280723 nonn,frac
%O A280723 0,1
%A A280723 _Ralf Steiner_, Jan 14 2017