a(n) = 4*a(n-1) + a(n-2), n>=2, a(0) = 2, a(1) = 7.

Terms

a(0) =2a(1) =7a(2) =30a(3) =127a(4) =538a(5) =2279a(6) =9654a(7) =40895a(8) =173234a(9) =733831a(10) =3108558a(11) =13168063a(12) =55780810a(13) =236291303a(14) =1000946022a(15) =4240075391a(16) =17961247586a(17) =76085065735a(18) =322301510526