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A229885 Number of 4 up, 4 down permutations of [n]. 3
1, 1, 1, 1, 1, 1, 5, 15, 35, 70, 574, 2674, 9274, 26599, 305747, 1944033, 8995805, 33757360, 498851248, 4017418768, 23236611280, 107709888805, 1945409895065, 18965460022971, 131635127294783, 726401013530416, 15505381392117616, 177447751441161616 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,7

COMMENTS

Limit n->infinity (a(n)/n!)^(1/n) = 0.38605986196... . - Vaclav Kotesovec, Sep 06 2014

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..200

EXAMPLE

a(5) = 1: 12345.

a(6) = 5: 123465, 123564, 124563, 134562, 234561.

a(7) = 15: 1234765, 1235764, 1236754, 1245763, 1246753, 1256743, 1345762, 1346752, 1356742, 1456732, 2345761, 2346751, 2356741, 2456731, 3456721.

MAPLE

b:= proc(u, o, t) option remember; `if`(u+o=0, 1, add(`if`(t=4,

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

    end:

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

seq(a(n), n=0..30);

CROSSREFS

Column k=4 of A229892.

Cf. A005983.

Sequence in context: A100355 A048032 A019499 * A243739 A145133 A270784

Adjacent sequences:  A229882 A229883 A229884 * A229886 A229887 A229888

KEYWORD

nonn,eigen

AUTHOR

Alois P. Heinz, Oct 02 2013

STATUS

approved

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Last modified January 29 14:07 EST 2020. Contains 331338 sequences. (Running on oeis4.)