Numbers k such that 7*R_(k+2) + 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

A257034

Numbers k such that 7*R_(k+2) + 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Terms

    a(0) =0a(1) =1a(2) =3a(3) =9a(4) =16a(5) =30a(6) =45a(7) =123a(8) =171a(9) =295a(10) =324a(11) =2785a(12) =2791a(13) =3910a(14) =4015a(15) =4050a(16) =6319a(17) =6415a(18) =14670

External references