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 A217213 #8 Oct 20 2014 17:15:17 %S A217213 0,0,0,4,30,168,840,3960,18018,80080,350064,1511640,6466460,27457584, %T A217213 115892400,486748080,2035917450,8485840800,35263382880,146157442200, %U A217213 604404010980,2494365759600,10275832148400,42264944401680,173588164506900,712027089322848,2917167840871200,11938779496898800 %N A217213 2*A002740(n). %H A217213 Luca Ferrari and Emanuele Munarini, <a href="http://arxiv.org/abs/1203.6807">Enumeration of saturated chains in Dyck lattices</a>, arXiv preprint arXiv:1203.6807, 2012 %Y A217213 Cf. A002740. %K A217213 nonn %O A217213 0,4 %A A217213 _N. J. A. Sloane_, Oct 03 2012