a(n) = 3*a(n-1) - a(n-2) - 3*a(n-3) + 2*a(n-4).

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =19a(5) =40a(6) =82a(7) =167a(8) =337a(9) =678a(10) =1360a(11) =2725a(12) =5455a(13) =10916a(14) =21838a(15) =43683a(16) =87373a(17) =174754a(18) =349516a(19) =699041a(20) =1398091a(21) =2796192a(22) =5592394a(23) =11184799a(24) =22369609a(25) =44739230a(26) =89478472a(27) =178956957a(28) =357913927a(29) =715827868