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

Terms

a(0) =1a(1) =2a(2) =6a(3) =8a(4) =9a(5) =11a(6) =20a(7) =23a(8) =41a(9) =63a(10) =66a(11) =119a(12) =122a(13) =149a(14) =252a(15) =284a(16) =305a(17) =592a(18) =746a(19) =875a(20) =1204a(21) =1364a(22) =2240a(23) =2403a(24) =5106a(25) =5776a(26) =5813a(27) =12456a(28) =14235a(29) =39606