Prime factors of the odd terms of A007755.

Terms

a(0) =3a(1) =5a(2) =11a(3) =17a(4) =41a(5) =83a(6) =137a(7) =257a(8) =641a(9) =1097a(10) =17477a(11) =65537a(12) =140417a(13) =557057a(14) =2384897a(15) =4227137a(16) =35946497a(17) =71304257a(18) =541073537a(19) =2281701377