Numbers k such that 4*5^k - 1 is prime.

Terms

a(0) =0a(1) =1a(2) =3a(3) =9a(4) =13a(5) =15a(6) =25a(7) =39a(8) =69a(9) =165a(10) =171a(11) =209a(12) =339a(13) =2033a(14) =6583a(15) =15393a(16) =282989a(17) =498483a(18) =504221a(19) =754611a(20) =864751