a(n) = (2*n)^(n-2)*(2*n)!/n!.A375105a(n) = (2*n)^(n-2)*(2*n)!/n!.Terms a(0) =1a(1) =12a(2) =720a(3) =107520a(4) =30240000a(5) =13795246080External referencesoeis: A375105