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%I #7 Apr 28 2014 19:00:54
%S 3,21,101,373,1174,3337,8642,21148,48713,108147,229984,476430,955667,
%T 1879169,3603751,6801106,12584048,22974152,41268291,73301654,
%U 128441712,222871187,382322022,650209758,1094918922,1829777544,3031447986,4987910871,8144753275
%N Number of ballot sequences of length n having exactly two descents.
%e a(5) = 3: [1,2,1,2,1], [1,2,1,3,1], [1,2,1,3,2].
%Y Column k=2 of A238121 and of A238122.
%K nonn
%O 5,1
%A _Joerg Arndt_ and _Alois P. Heinz_, Apr 28 2014