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 A281070 #35 Jan 15 2017 13:22:07
%S A281070 3,9,109,1037,91027,1540981,447810157,147053171,503445581741,
%T A281070 16337573574319,88973047698967,3588920671411951,2314594755016141847,
%U A281070 20685050199210758743,2160689714871889935101,121435710295138581181033,16427863327419202412927713
%N A281070 a(n) is the numerator of 6 * Sum_{k=0..n} ((k+1)/(n-k+1)^2) * (Catalan(k)/(2^(2*k+1)))^2.
%C A281070 The series a(n)/A280723(n) is absolutely convergent to Pi.
%t A281070 a[n_]=6(Sum[(1/(n-k+1)^2)((CatalanNumber[k])/(2^(2k+1)))^2(k+1),{k,0, n}]); Numerator /@a/@ Range[0,10]
%Y A281070 Cf. A000108 (Catalan), A280723 (denominators).
%K A281070 nonn,frac
%O A281070 0,1
%A A281070 _Ralf Steiner_, Jan 14 2017