

A193776


Number of signed permutations of length n invariant under the reverse complement and avoiding (2, 1), (2, 1), (2, 1).


0




OFFSET

0,2


LINKS

Table of n, a(n) for n=0..9.


FORMULA

a(2k) = 2^k k! + \sum_{j=0}^{k1}(kj1)! a(2j)
a(2k+1) = (2^k + 1)k! + \sum_{j=0}^{k1}(kj1)! a(2j+1)


EXAMPLE

For n = 2, the permutations are (1, 2), (2, 1), (1, 2), (2, 1).


CROSSREFS

Sequence in context: A158936 A002139 A140489 * A051915 A064688 A089891
Adjacent sequences: A193773 A193774 A193775 * A193777 A193778 A193779


KEYWORD

nonn


AUTHOR

Andy Hardt, Aug 04 2011


STATUS

approved



