Numbers k such that 7*R_(k+2) - 5*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

A257028

Numbers k such that 7*R_(k+2) - 5*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Terms

    a(0) =1a(1) =4a(2) =10a(3) =28a(4) =49a(5) =64a(6) =79a(7) =109a(8) =169a(9) =1270a(10) =5638a(11) =6812a(12) =7951a(13) =11737a(14) =16360a(15) =22840a(16) =25394a(17) =33394

External references