Odd Mersenne exponents.

Terms

a(0) =3a(1) =5a(2) =7a(3) =13a(4) =17a(5) =19a(6) =31a(7) =61a(8) =89a(9) =107a(10) =127a(11) =521a(12) =607a(13) =1279a(14) =2203a(15) =2281a(16) =3217a(17) =4253a(18) =4423a(19) =9689a(20) =9941a(21) =11213a(22) =19937a(23) =21701a(24) =23209a(25) =44497a(26) =86243a(27) =110503a(28) =132049a(29) =216091