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A227941 Number of 1 up, 3 down, 5 up, 7 down, ... permutations of [n]. 4
1, 1, 1, 2, 3, 4, 19, 55, 125, 245, 434, 4060, 21186, 81212, 254813, 692678, 1688555, 3776432, 60101767, 511650887, 3089821383, 14824989723, 60057570858, 213302293918, 681247718668, 1992449334436, 5409214694961, 132273848506202, 1692162553490943 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..300

EXAMPLE

a(2) = 1: 12.

a(3) = 2: 132, 231.

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

a(5) = 4: 15432, 25431, 35421, 45321.

a(6) = 19: 154326, 164325, 165324, 165423, 254316, 264315, 265314, 265413, 354216, 364215, 365214, 365412, 453216, 463215, 465213, 465312, 563214, 564213, 564312.

a(7) = 55: 1543267, 1643257, ..., 6753124, 6754123.

a(8) = 125: 15432678, 16432578, ..., 78641235, 78651234.

MAPLE

b:= proc(u, o, t, k) option remember; `if`(u+o=0, 1, add(`if`(t=2*k-1,

       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. A005408, A229066, A229551, A229892.

Sequence in context: A067805 A092837 A058772 * A250259 A276105 A247574

Adjacent sequences:  A227938 A227939 A227940 * A227942 A227943 A227944

KEYWORD

nonn,eigen

AUTHOR

Alois P. Heinz, Oct 03 2013

STATUS

approved

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Last modified September 28 18:04 EDT 2021. Contains 347716 sequences. (Running on oeis4.)