Numbers k such that 7*10^k + 5*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

A103060

Numbers k such that 7*10^k + 5*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Terms

    a(0) =1a(1) =2a(2) =5a(3) =7a(4) =23a(5) =32a(6) =38a(7) =47a(8) =116a(9) =140a(10) =229a(11) =247a(12) =734a(13) =931a(14) =1346a(15) =2162a(16) =2546a(17) =6980a(18) =10283a(19) =10543a(20) =12128a(21) =13741a(22) =105320

External references