If n mod 4 = 0 then 2^(n-1)+1 elif n mod 4 = 2 then 2^(n-1)-1 else 2^(n-1).

Terms

a(0) =1a(1) =1a(2) =4a(3) =9a(4) =16a(5) =31a(6) =64a(7) =129a(8) =256a(9) =511a(10) =1024a(11) =2049a(12) =4096a(13) =8191a(14) =16384a(15) =32769a(16) =65536a(17) =131071a(18) =262144a(19) =524289a(20) =1048576a(21) =2097151a(22) =4194304a(23) =8388609a(24) =16777216a(25) =33554431a(26) =67108864a(27) =134217729a(28) =268435456a(29) =536870911