Numbers k such that 9*R_(k+2) - 7*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

A257037

Numbers k such that 9*R_(k+2) - 7*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Terms

    a(0) =1a(1) =8a(2) =9a(3) =14a(4) =54a(5) =80a(6) =487a(7) =551a(8) =600a(9) =2502a(10) =2544a(11) =5593a(12) =7949a(13) =8635a(14) =13407a(15) =31128a(16) =45504a(17) =45933a(18) =52303a(19) =65121a(20) =167501a(21) =359354a(22) =642225a(23) =1029523a(24) =1170023

External references