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 A256724 Numbers k such that R_k + 30 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
 1, 2, 8, 56, 675470, 718580 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, numbers k such that (10^k + 269)/9 is prime. Terms from Kamada data. Note that Kamada does not recognize k=1 as 31 is a degenerate case of form AAA..ABA. a(7) > 10^6. LINKS Makoto Kamada, Near-repdigit numbers of the form AA...AABA. Makoto Kamada, Prime numbers of the form 11...1141. EXAMPLE For k=8, R_8 + 30 = 11111111 + 30 = 11111141 which is prime. MATHEMATICA Select[Range[0, 250000], PrimeQ[(10^# + 269)/9] &] PROG (MAGMA) [n: n in [1..400] | IsPrime((10^n+269) div 9)]; // Vincenzo Librandi, Apr 10 2015 CROSSREFS Cf. A002275. Sequence in context: A191508 A113725 A345677 * A153538 A153566 A153529 Adjacent sequences:  A256721 A256722 A256723 * A256725 A256726 A256727 KEYWORD more,hard,nonn AUTHOR Robert Price, Apr 09 2015 STATUS approved

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Last modified August 8 17:44 EDT 2022. Contains 356016 sequences. (Running on oeis4.)