a(n) = ((1 + sqrt(7))^n - (1 - sqrt(7))^n)/sqrt(7).

Terms

a(0) =0a(1) =2a(2) =4a(3) =20a(4) =64a(5) =248a(6) =880a(7) =3248a(8) =11776a(9) =43040a(10) =156736a(11) =571712a(12) =2083840a(13) =7597952a(14) =27698944a(15) =100985600a(16) =368164864a(17) =1342243328a(18) =4893475840a(19) =17840411648a(20) =65041678336a(21) =237125826560a(22) =864501723136