%I #6 Apr 28 2014 19:01:18
%S 1,17,145,836,3846,15002,52132,164576,484609,1337597,3521517,8850458,
%T 21485321,50358968,114911640,255114977,554333346,1178070741,
%U 2459840199,5042845917,10185374250,20257833206,39778823250,77088115856,147739793527,279923493220
%N Number of ballot sequences of length n having exactly three descents.
%e a(6) = 1: [1,2,1,3,2,1].
%Y Column k=3 of A238121 and of A238122.
%K nonn
%O 6,2
%A _Joerg Arndt_ and _Alois P. Heinz_, Apr 28 2014