Smallest prime dividing 2^n - 1.

Terms

a(0) =3a(1) =7a(2) =3a(3) =31a(4) =3a(5) =127a(6) =3a(7) =7a(8) =3a(9) =23a(10) =3a(11) =8191a(12) =3a(13) =7a(14) =3a(15) =131071a(16) =3a(17) =524287a(18) =3a(19) =7a(20) =3a(21) =47a(22) =3a(23) =31a(24) =3a(25) =7a(26) =3a(27) =233a(28) =3a(29) =2147483647