a(n) is the smallest prime p such that p^2 - 1 has 2*n divisors, or -1 if no such prime exists.

Terms

a(0) =2a(1) =3a(2) =-1a(3) =5a(4) =7a(5) =-1a(6) =-1a(7) =11a(8) =17a(9) =23a(10) =-1a(11) =19a(12) =-1a(13) =31a(14) =73a(15) =29a(16) =-1a(17) =383a(18) =-1a(19) =41a(20) =97a(21) =-1a(22) =-1a(23) =79a(24) =-1a(25) =-1a(26) =127a(27) =223a(28) =-1a(29) =71