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

A056715

Numbers k such that 5*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) =8a(3) =14a(4) =17a(5) =18a(6) =33a(7) =35a(8) =126a(9) =183a(10) =324a(11) =344a(12) =866a(13) =992a(14) =1226a(15) =2355a(16) =13344a(17) =75027a(18) =77322

External references