a(n) = (2n)!/2^n.A000680a(n) = (2n)!/2^n.Terms a(0) =1a(1) =1a(2) =6a(3) =90a(4) =2520a(5) =113400a(6) =7484400a(7) =681080400a(8) =81729648000External referencesoeis: A000680