Numbers k such that 4*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

A056708

Numbers k such that 4*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Terms

    a(0) =1a(1) =4a(2) =13a(3) =25a(4) =36a(5) =357a(6) =373a(7) =1041a(8) =1089a(9) =1093a(10) =1297a(11) =8274a(12) =10732a(13) =15972a(14) =18114a(15) =21823a(16) =34519a(17) =36096a(18) =75498

External references