Numbers k such that (10^k)/2 - 1 is prime.

Terms

a(0) =3a(1) =4a(2) =5a(3) =7a(4) =15a(5) =55a(6) =211a(7) =391a(8) =595a(9) =3461a(10) =5029a(11) =5220a(12) =5333a(13) =8073a(14) =15797a(15) =16132a(16) =21457a(17) =29283a(18) =78791a(19) =85143a(20) =179973a(21) =211030a(22) =445774a(23) =464844a(24) =511057