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A229066 Number of 1 up, 2 down, 3 up, 4 down, ... permutations of [n]. 4
1, 1, 1, 2, 3, 11, 26, 50, 315, 1168, 3309, 7910, 78134, 431354, 1748956, 5797168, 16619603, 239887424, 1875375485, 10496708022, 47013492080, 178807998112, 599025922320, 11965846097382, 126883998286089, 947079890934441, 5574231845278396, 27500583638094490 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..250

EXAMPLE

a(2) = 1: 12.

a(3) = 2: 132, 231.

a(4) = 3: 1432, 2431, 3421.

a(5) = 11: 14325, 15324, 15423, 24315, 25314, 25413, 34215, 35214, 35412, 45213, 45312.

a(6) = 26: 143256, 153246, ..., 563124, 564123.

a(7) = 50: 1432567, 1532467, ..., 6741235, 6751234.

a(8) = 315: 14325687, 14325786, ..., 78613452, 78623451.

MAPLE

b:= proc(u, o, t, k) option remember; `if`(u+o=0, 1, add(`if`(t=k,

       b(o-j, u+j-1, 1, k+1), b(u+j-1, o-j, t+1, k)), j=1..o))

    end:

a:= n-> b(0, n, 0, 1):

seq(a(n), n=0..35);

CROSSREFS

Cf. A227941, A229551, A229892.

Sequence in context: A239445 A157161 A041811 * A248161 A056851 A284046

Adjacent sequences:  A229063 A229064 A229065 * A229067 A229068 A229069

KEYWORD

nonn,eigen

AUTHOR

Alois P. Heinz, Oct 02 2013

STATUS

approved

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Last modified April 19 06:30 EDT 2019. Contains 322237 sequences. (Running on oeis4.)