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A193776 Number of signed permutations of length n invariant under the reverse complement and avoiding (-2, -1), (-2, 1), (2, -1). 0
1, 2, 3, 5, 12, 17, 65, 80, 473, 527 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

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

FORMULA

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

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

EXAMPLE

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

CROSSREFS

Sequence in context: A002139 A140489 A331101 * A051915 A064688 A089891

Adjacent sequences:  A193773 A193774 A193775 * A193777 A193778 A193779

KEYWORD

nonn

AUTHOR

Andy Hardt, Aug 04 2011

STATUS

approved

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Last modified January 20 14:02 EST 2020. Contains 331094 sequences. (Running on oeis4.)