A241805 Number of ballot sequences of length 2n+1 having exactly n descents.

1, 1, 3, 17, 146, 1615, 21895, 349021, 6385041, 131722015, 3019039806, 76052414727, 2087753039823, 61995921313948, 1979138443335708, 67572818967182165, 2456489615222703765, 94710044795486692712, 3859301582400404494533, 165700769713109075755060

Offset: 0

Author

Joerg Arndt and Alois P. Heinz, Apr 28 2014

Keywords

• nonn

Examples

			a(1) = 1: [1,2,1].
a(2) = 3: [1,2,1,2,1], [1,2,1,3,1], [1,2,1,3,2].
		

Formula

a(n) = A238121(2n+1,n) = A238122(2n+1,n).