a(n) = (3^(n-1) - 1) * (3^n - 1)/2.

Terms

a(0) =0a(1) =8a(2) =104a(3) =1040a(4) =9680a(5) =88088a(6) =795704a(7) =7170080a(8) =64556960a(9) =581091368a(10) =5230058504a(11) =47071235120a(12) =423643241840