a(n) = (4^(n + 1) - 2)*(2*n)!.A331839a(n) = (4^(n + 1) - 2)*(2*n)!.Terms a(0) =2a(1) =28a(2) =1488a(3) =182880a(4) =41207040a(5) =14856307200External referencesoeis: A331839