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

A056702

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

Terms

    a(0) =0a(1) =2a(2) =3a(3) =9a(4) =15a(5) =18a(6) =36a(7) =63a(8) =114a(9) =225a(10) =405a(11) =482a(12) =1241a(13) =2018a(14) =9830a(15) =15795a(16) =22535a(17) =27752a(18) =41033a(19) =95574a(20) =131144a(21) =156221

External references