Numbers k such that 7*R_(k+2) + 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
A257033
Numbers k such that 7*R_(k+2) + 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
Terms
- a(0) =1a(1) =2a(2) =4a(3) =5a(4) =7a(5) =11a(6) =25a(7) =31a(8) =109a(9) =205a(10) =518a(11) =1288a(12) =1697a(13) =1711a(14) =1775a(15) =1997a(16) =6784a(17) =8741a(18) =13171a(19) =20275a(20) =67517
External references
- oeis: A257033