a(n) = Sum_{k=2..n} 8^k.

Terms

a(0) =0a(1) =64a(2) =576a(3) =4672a(4) =37440a(5) =299584a(6) =2396736a(7) =19173952a(8) =153391680a(9) =1227133504a(10) =9817068096a(11) =78536544832a(12) =628292358720