Numbers k such that 7*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

A056720

Numbers k such that 7*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Terms

    a(0) =0a(1) =1a(2) =2a(3) =3a(4) =5a(5) =53a(6) =56a(7) =343a(8) =908a(9) =1079a(10) =2204a(11) =2379a(12) =9134a(13) =9371a(14) =9728a(15) =11039a(16) =11247a(17) =13275a(18) =16781a(19) =20138a(20) =75368a(21) =87863

External references