Numbers k such that 5*10^k + 2*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

A103009

Numbers k such that 5*10^k + 2*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Terms

    a(0) =1a(1) =2a(2) =4a(3) =7a(4) =103a(5) =166a(6) =280a(7) =475a(8) =505a(9) =835a(10) =922a(11) =2018a(12) =4435a(13) =4459a(14) =7615a(15) =8558a(16) =17164a(17) =22801

External references