a(n) is the least prime factor > 3 of 10^n - 1.

Terms

a(0) =11a(1) =37a(2) =11a(3) =41a(4) =7a(5) =239a(6) =11a(7) =37a(8) =11a(9) =21649a(10) =7a(11) =53a(12) =11a(13) =31a(14) =11a(15) =2071723a(16) =7a(18) =11a(19) =37a(20) =11a(22) =7a(23) =41a(24) =11a(25) =37a(26) =11a(27) =3191a(28) =7a(29) =2791a(30) =11a(31) =37