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A241871
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Number of ascent sequences of length 2n with exactly n descents.
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2
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1, 0, 0, 1, 26, 937, 45747, 2945144, 242899690, 25034354941, 3157647587689, 478931493603308, 86057396214591300, 18086772915953351382, 4397414569504319733812, 1224945090342466220614714, 387654163770235904289085798, 138333762956844287480268151988
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OFFSET
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0,5
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LINKS
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FORMULA
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MAPLE
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b:= proc(n, i, t) option remember; `if`(n=0, 1, expand(add(
`if`(j<i, x, 1) *b(n-1, j, t+`if`(j>i, 1, 0)), j=0..t+1)))
end:
a:= n-> coeff(b(2*n, -1$2), x, n):
seq(a(n), n=0..20);
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MATHEMATICA
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b[n_, i_, t_] := b[n, i, t] = If[n == 0, 1, Expand[Sum[If[j<i, x, 1]*b[n-1, j, t + If[j>i, 1, 0]], {j, 0, t+1}]]]; a[n_] := Coefficient[b[2*n, -1, -1], x, n]; Table[ a[n], {n, 0, 20}] (* Jean-François Alcover, Feb 13 2015, after Maple *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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