a(n) = 3*a(n-1) - 3*a(n-2) + 2*a(n-3), with a(0) = 4, a(1) = 2, a(2) = 1.

Terms

a(0) =4a(1) =2a(2) =1a(3) =5a(4) =16a(5) =35a(6) =67a(7) =128a(8) =253a(9) =509a(10) =1024a(11) =2051a(12) =4099a(13) =8192a(14) =16381a(15) =32765a(16) =65536a(17) =131075a(18) =262147a(19) =524288a(20) =1048573a(21) =2097149a(22) =4194304a(23) =8388611a(24) =16777219a(25) =33554432a(26) =67108861a(27) =134217725a(28) =268435456a(29) =536870915