a(n) = 2^n + sin(n*Pi/2).

Terms

a(0) =1a(1) =3a(2) =4a(3) =7a(4) =16a(5) =33a(6) =64a(7) =127a(8) =256a(9) =513a(10) =1024a(11) =2047a(12) =4096a(13) =8193a(14) =16384a(15) =32767a(16) =65536a(17) =131073a(18) =262144a(19) =524287a(20) =1048576a(21) =2097153a(22) =4194304a(23) =8388607a(24) =16777216a(25) =33554433a(26) =67108864a(27) =134217727a(28) =268435456a(29) =536870913