Second smallest prime after 2^n.

Terms

a(0) =3a(1) =5a(2) =7a(3) =13a(4) =19a(5) =41a(6) =71a(7) =137a(8) =263a(9) =523a(10) =1033a(11) =2063a(12) =4111a(13) =8219a(14) =16417a(15) =32779a(16) =65539a(17) =131111a(18) =262151a(19) =524341a(20) =1048589a(21) =2097211a(22) =4194329a(23) =8388619a(24) =16777289a(25) =33554473a(26) =67108913a(27) =134217773a(28) =268435463a(29) =536870951