a(n) = (9*2^n + (-2)^n)/4 for n>0.

Terms

a(0) =4a(1) =10a(2) =16a(3) =40a(4) =64a(5) =160a(6) =256a(7) =640a(8) =1024a(9) =2560a(10) =4096a(11) =10240a(12) =16384a(13) =40960a(14) =65536a(15) =163840a(16) =262144a(17) =655360a(18) =1048576a(19) =2621440a(20) =4194304a(21) =10485760a(22) =16777216a(23) =41943040a(24) =67108864a(25) =167772160a(26) =268435456a(27) =671088640a(28) =1073741824a(29) =2684354560