Numbers k such that 7*10^k + 4*R_k - 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
A103058
Numbers k such that 7*10^k + 4*R_k - 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
Terms
- a(0) =1a(1) =2a(2) =19a(3) =31a(4) =106a(5) =26194a(6) =39973
External references
- oeis: A103058