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 A067000 #3 Mar 30 2012 16:49:18 %S A067000 1,3,27,339,5553,112761,2736099,77252175,2488315257,90053238033, %T A067000 3617791773723,159761716507215,7692231450102849,401057043646065345, %U A067000 22510943881422758739,1353385233080839510335,86771327488839208390473 %N A067000 a(1) = 1; a(n) = Sum (n+k)!a(k)/(2k)!, k = 1..n-1. %Y A067000 Cf. A007683. %K A067000 nonn %O A067000 1,2 %A A067000 _N. J. A. Sloane_, Feb 16 2002