a(n) = 2^n + 3.

Terms

a(0) =4a(1) =5a(2) =7a(3) =11a(4) =19a(5) =35a(6) =67a(7) =131a(8) =259a(9) =515a(10) =1027a(11) =2051a(12) =4099a(13) =8195a(14) =16387a(15) =32771a(16) =65539a(17) =131075a(18) =262147a(19) =524291a(20) =1048579a(21) =2097155a(22) =4194307a(23) =8388611a(24) =16777219a(25) =33554435a(26) =67108867a(27) =134217731a(28) =268435459a(29) =536870915