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A302904
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Number of permutations of [2n] having exactly n-1 alternating descents.
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3
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1, 1, 7, 182, 8699, 704834, 84889638, 14322115212, 3216832016019, 928559550102410, 334876925319944690, 147563833511292247796, 78009671642511668089822, 48728981875112003682759892, 35506576774281843111748649644, 29848802048200930275501944893080
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OFFSET
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0,3
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COMMENTS
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a(0) = 1 by convention.
Index i is an alternating descent of permutation p if either i is odd and p(i) > p(i+1), or i is even and p(i) < p(i+1).
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LINKS
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FORMULA
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a(n) ~ sqrt(3) * 2^(2*n + 1) * n^(2*n) / (sqrt(5) * exp(2*n)). - Vaclav Kotesovec, Apr 29 2018
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EXAMPLE
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a(2) = 7: 1234, 1432, 2431, 3214, 3421, 4213, 4312.
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MAPLE
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b:= proc(u, o) option remember; expand(`if`(u+o=0, 1,
add(b(o+j-1, u-j)*x, j=1..u)+
add(b(o-j, u-1+j), j=1..o)))
end:
a:= n-> coeff(b(2*n, 0), x, n):
seq(a(n), n=0..20);
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MATHEMATICA
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b[u_, o_] := b[u, o] = Expand[If[u + o == 0, 1,
Sum[b[o + j - 1, u - j] x, {j, 1, u}] +
Sum[b[o - j, u - 1 + j], {j, 1, o}]]];
a[n_] := Coefficient[b[2 n, 0], x, n];
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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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