a(n) = (9^n - 1)/2.

Terms

a(0) =0a(1) =4a(2) =40a(3) =364a(4) =3280a(5) =29524a(6) =265720a(7) =2391484a(8) =21523360a(9) =193710244a(10) =1743392200a(11) =15690529804a(12) =141214768240