a(n) = 6*a(n-1) - a(n-2).

Terms

a(0) =0a(1) =4a(2) =24a(3) =140a(4) =816a(5) =4756a(6) =27720a(7) =161564a(8) =941664a(9) =5488420a(10) =31988856a(11) =186444716a(12) =1086679440a(13) =6333631924a(14) =36915112104a(15) =215157040700