Numbers k such that (5^k - 1)/4 is prime.

Terms

a(0) =3a(1) =7a(2) =11a(3) =13a(4) =47a(5) =127a(6) =149a(7) =181a(8) =619a(9) =929a(10) =3407a(11) =10949a(12) =13241a(13) =13873a(14) =16519a(15) =201359a(16) =396413a(17) =1888279a(18) =3300593