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A241804
Number of ballot sequences of length 2n having exactly n descents.
2
1, 0, 0, 1, 9, 112, 1582, 26159, 486170, 10131071, 233582056, 5895589738, 161776346286, 4794754890798, 152576687930997, 5187562649814737, 187666786444911220, 7196725552061993554, 291572346138097726746, 12443163766976461627878, 557880276243757014820691
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OFFSET
0,5
LINKS
Table of n, a(n) for n=0..20.
FORMULA
a(n) =
A238121
(2n,n) =
A238122
(2n,n).
EXAMPLE
a(3) = 1: [1,2,1,3,2,1].
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].
CROSSREFS
Sequence in context:
A201532
A180788
A348068
*
A156949
A155624
A165224
Adjacent sequences:
A241801
A241802
A241803
*
A241805
A241806
A241807
KEYWORD
nonn
AUTHOR
Joerg Arndt
and
Alois P. Heinz
, Apr 28 2014
STATUS
approved
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Last modified May 12 08:55 EDT 2024. Contains 372432 sequences. (Running on oeis4.)