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A193775
Number of signed permutations invariant under the reverse complement and avoiding (-2, -1), (-1, 2), and (1, -2).
0
1, 2, 3, 4, 10, 12, 52, 58, 394, 418
OFFSET
0,2
FORMULA
a(n) = (\lfloor n/2\rfloor)! 2^{\lfloor n/2\rfloor} + \sum_{k=0}^{\lfloor n/2\lceil} k!
EXAMPLE
For n = 3, the permutations are (1, 2, 3), (3, 2, 1), (-1, -2, -3), and (-3, 2, -1).
CROSSREFS
Sequence in context: A005456 A100773 A250112 * A131120 A115195 A095384
KEYWORD
nonn
AUTHOR
Andy Hardt, Aug 04 2011
STATUS
approved