

A129815


Number of reverse alternating fixedpointfree permutations on n letters.


4



0, 0, 1, 2, 6, 22, 102, 506, 2952, 18502, 131112, 991226, 8271792, 73176262, 703077552, 7121578106, 77437418112, 883521487942, 10726837356672, 136104948161786, 1825110309733632
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OFFSET

1,4


COMMENTS

From Emeric Deutsch, Aug 06 2009: (Start)
Reverse alternating permutations are called also updown permutations.
a(n) is also the number of reverse alternating permutations having exactly 1 fixed point (see the Stanley reference). Example: a(4)=2 because we have 1423 and 2314.
(End)


LINKS

Table of n, a(n) for n=1..21.
R. P. Stanley, Alternating permutations and symmetric functions


FORMULA

a(2n1) = A129817(2n1). [Emeric Deutsch, Aug 06 2009]


EXAMPLE

a(4)=2 because we have 3412 and 2413. [Emeric Deutsch, Aug 06 2009]


CROSSREFS

Cf. A000111, A000166, A007779.
Sequence in context: A002772 A000140 A079263 * A251181 A309579 A103941
Adjacent sequences: A129812 A129813 A129814 * A129816 A129817 A129818


KEYWORD

more,nonn


AUTHOR

Vladeta Jovovic, May 20 2007


EXTENSIONS

a(21) from Alois P. Heinz, Jun 11 2015


STATUS

approved



