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
KEYWORD
nonn
AUTHOR
Andy Hardt, Aug 04 2011
STATUS
approved