Numbers k such that 9*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

A056727

Numbers k such that 9*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Terms

    a(0) =1a(1) =2a(2) =4a(3) =19a(4) =28a(5) =73a(6) =203a(7) =220a(8) =274a(9) =292a(10) =470a(11) =763a(12) =1891a(13) =3307a(14) =7007a(15) =7306a(16) =9755a(17) =11395a(18) =39452a(19) =78242

External references