Numbers n such that 5*10^n + 7*R_n - 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
A103020
Numbers n such that 5*10^n + 7*R_n - 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
Terms
- a(0) =1a(1) =4a(2) =12a(3) =15a(4) =18a(5) =37a(6) =109a(7) =1728a(8) =2482a(9) =3480a(10) =6577a(11) =9015a(12) =16162a(13) =45349
External references
- oeis: A103020