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A302903 Number of permutations of [2n+1] having exactly n alternating descents. 3
1, 2, 36, 1196, 76840, 7570716, 1085246904, 211595659320, 53984412657360, 17440458896525180, 6960292943873805976, 3362366089440205308072, 1933633403768889597292336, 1305355624659052356741634136, 1022196734801743485304805455920, 919035074839303194470918726496240 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

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).

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..100

D. Chebikin, Variations on descents and inversions in permutations, The Electronic J. of Combinatorics, 15 (2008), #R132.

FORMULA

a(n) = A145876(2n+1,n+1) = A302905(2n+1).

a(n) ~ sqrt(3) * 2^(2*n + 2) * n^(2*n + 1) / (sqrt(5) * exp(2*n)). - Vaclav Kotesovec, Apr 29 2018

EXAMPLE

a(1) = 2: 123, 321.

a(2) = 36: 12345, 12543, 13542, 14325, 14532, 15324, 15423, 21354, 21453, 23541, 24315, 24531, 25314, 25413, 31254, 31452, 32145, 32451, 34215, 34521, 35214, 35412, 41253, 41352, 42135, 42351, 43125, 45213, 45312, 51243, 51342, 52134, 52341, 53124, 54123, 54321.

MAPLE

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+1, 0), x, n+1):

seq(a(n), n=0..20);

MATHEMATICA

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[2n + 1, 0], x, n + 1];

a /@ Range[0, 20] (* Jean-François Alcover, Dec 21 2020, after Alois P. Heinz *)

CROSSREFS

Bisection (odd part) of A302905.

Cf. A145876.

Sequence in context: A209036 A305596 A210899 * A259467 A003092 A187535

Adjacent sequences:  A302900 A302901 A302902 * A302904 A302905 A302906

KEYWORD

nonn

AUTHOR

Alois P. Heinz, Apr 15 2018

STATUS

approved

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Last modified December 6 10:12 EST 2021. Contains 349563 sequences. (Running on oeis4.)