a(n) = ((1 + sqrt(11))^n - (1 - sqrt(11))^n)/sqrt(11).

Terms

a(0) =0a(1) =2a(2) =4a(3) =28a(4) =96a(5) =472a(6) =1904a(7) =8528a(8) =36096a(9) =157472a(10) =675904a(11) =2926528a(12) =12612096a(13) =54489472a(14) =235099904a(15) =1015094528a(16) =4381188096a(17) =18913321472a(18) =81638523904a(19) =352410262528