a(n) = (2*n+1) * (3*n+1)^(n-2) * 4^n.A251694a(n) = (2*n+1) * (3*n+1)^(n-2) * 4^n.Terms a(0) =1a(1) =3a(2) =80a(3) =4480a(4) =389376a(5) =46137344a(6) =6939332608External referencesoeis: A251694