a(n) = (2*n-1)! + (2*n)!.A174549a(n) = (2*n-1)! + (2*n)!.Terms a(0) =3a(1) =30a(2) =840a(3) =45360a(4) =3991680a(5) =518918400a(6) =93405312000External referencesoeis: A174549