a(0) = 1, a(1) = 3; for n > 1, a(n) = 2*a(n-1) + 4*a(n-2).

Terms

a(0) =1a(1) =3a(2) =10a(3) =32a(4) =104a(5) =336a(6) =1088a(7) =3520a(8) =11392a(9) =36864a(10) =119296a(11) =386048a(12) =1249280a(13) =4042752a(14) =13082624a(15) =42336256a(16) =137003008a(17) =443351040a(18) =1434714112a(19) =4642832384a(20) =15024521216a(21) =48620371968a(22) =157338828800a(23) =509159145472