Numbers k such that 6*R_k - 5 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Terms

a(0) =2a(1) =3a(2) =4a(3) =10a(4) =18a(5) =21a(6) =22a(7) =28a(8) =43a(9) =66a(10) =121a(11) =133a(12) =178a(13) =241a(14) =454a(15) =553a(16) =1600a(17) =2175a(18) =2978a(19) =3649a(20) =7708a(21) =8316a(22) =10392a(23) =12458a(24) =21057a(25) =26223a(26) =48297a(27) =64041a(28) =84904a(29) =92976