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%I #6 Apr 28 2014 19:05:30
%S 1,1,3,17,146,1615,21895,349021,6385041,131722015,3019039806,
%T 76052414727,2087753039823,61995921313948,1979138443335708,
%U 67572818967182165,2456489615222703765,94710044795486692712,3859301582400404494533,165700769713109075755060
%N Number of ballot sequences of length 2n+1 having exactly n descents.
%F a(n) = A238121(2n+1,n) = A238122(2n+1,n).
%e a(1) = 1: [1,2,1].
%e a(2) = 3: [1,2,1,2,1], [1,2,1,3,1], [1,2,1,3,2].
%K nonn
%O 0,3
%A _Joerg Arndt_ and _Alois P. Heinz_, Apr 28 2014
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