a(n) = (2*n)!*(2*n-1)/(2^n*n!).

Terms

a(0) =-1a(1) =1a(2) =9a(3) =75a(4) =735a(5) =8505a(6) =114345a(7) =1756755a(8) =30405375a(9) =585810225a(10) =12439852425a(11) =288735522075