Primes of the form 2^r * 13^s - 1.

Terms

a(0) =3a(1) =7a(2) =31a(3) =103a(4) =127a(5) =337a(6) =1663a(7) =5407a(8) =8191a(9) =131071a(10) =346111a(11) =524287a(12) =2970343a(13) =3655807a(14) =22151167a(15) =109051903a(16) =617831551a(17) =1631461441a(18) =2007952543a(19) =2147483647a(20) =32127240703a(21) =194664464383a(22) =275716983697a(23) =958348132351