A193775 Number of signed permutations invariant under the reverse complement and avoiding (-2, -1), (-1, 2), and (1, -2).

1, 2, 3, 4, 10, 12, 52, 58, 394, 418

Offset: 0

Author

Andy Hardt, Aug 04 2011

Keywords

• nonn

Examples

			For n = 3, the permutations are (1, 2, 3), (3, 2, 1), (-1, -2, -3), and (-3, 2, -1).
		

Formula

a(n) = (\lfloor n/2\rfloor)! 2^{\lfloor n/2\rfloor} + \sum_{k=0}^{\lfloor n/2\lceil} k!