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 A241804 #7 Apr 28 2014 19:04:47 %S A241804 1,0,0,1,9,112,1582,26159,486170,10131071,233582056,5895589738, %T A241804 161776346286,4794754890798,152576687930997,5187562649814737, %U A241804 187666786444911220,7196725552061993554,291572346138097726746,12443163766976461627878,557880276243757014820691 %N A241804 Number of ballot sequences of length 2n having exactly n descents. %F A241804 a(n) = A238121(2n,n) = A238122(2n,n). %e A241804 a(3) = 1: [1,2,1,3,2,1]. %e A241804 a(4) = 9: [1,2,1,2,1,3,2,1], [1,2,1,3,1,4,2,1], [1,2,1,3,2,1,2,1], [1,2,1,3,2,1,3,1], [1,2,1,3,2,1,3,2], [1,2,1,3,2,1,4,1], [1,2,1,3,2,1,4,2], [1,2,1,3,2,1,4,3], [1,2,1,3,2,4,3,1]. %K A241804 nonn %O A241804 0,5 %A A241804 _Joerg Arndt_ and _Alois P. Heinz_, Apr 28 2014