OFFSET
1,2
FORMULA
a(n) = (n!)^2/(2*n)*sum{k = 0..n+1}((-1)^k/k!*binomial(2*n-2*k, n-k)*2^k).
EXAMPLE
When n=2, there are four people seated around a circular table. Person 1 can sit across from either person 2 or person 4, and person 3 can sit either to the left or to the right of person 1. Thus a(2) = 2*2=4.
PROG
(PARI) a(n) = n!^2/(2*n)*sum(k = 0, n+1, (-1)^k/k!*binomial(2*n-2*k, n-k)*2^k) \\ Michel Marcus, Jul 11 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Steven Klee (klees(AT)math.washington.edu), Nov 03 2009
EXTENSIONS
More terms from Michel Marcus, Jul 11 2013
STATUS
approved