a(n) = 4*a(n-1) - a(n-2) with a(0)=1, a(1)=8.

Terms

a(0) =1a(1) =8a(2) =31a(3) =116a(4) =433a(5) =1616a(6) =6031a(7) =22508a(8) =84001a(9) =313496a(10) =1169983a(11) =4366436a(12) =16295761a(13) =60816608a(14) =226970671a(15) =847066076a(16) =3161293633a(17) =11798108456a(18) =44031140191a(19) =164326452308a(20) =613274669041