a(n) = 2^n+(-1)^n-1.

Terms

a(0) =1a(1) =0a(2) =4a(3) =6a(4) =16a(5) =30a(6) =64a(7) =126a(8) =256a(9) =510a(10) =1024a(11) =2046a(12) =4096a(13) =8190a(14) =16384a(15) =32766a(16) =65536a(17) =131070a(18) =262144a(19) =524286a(20) =1048576a(21) =2097150a(22) =4194304a(23) =8388606a(24) =16777216a(25) =33554430a(26) =67108864a(27) =134217726a(28) =268435456a(29) =536870910