Primes of the form 2^r*5^s - 1.

Terms

a(0) =3a(1) =7a(2) =19a(3) =31a(4) =79a(5) =127a(6) =199a(7) =499a(8) =1249a(9) =1279a(10) =1999a(11) =4999a(12) =5119a(13) =8191a(14) =12799a(15) =20479a(16) =31249a(17) =49999a(18) =51199a(19) =79999a(20) =81919a(21) =131071a(22) =199999a(23) =524287a(24) =799999a(25) =1249999a(26) =1310719a(27) =3124999a(28) =3276799a(29) =4999999