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 A256726 Numbers k such that 7*R_k - 50 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
 3, 4, 7, 12, 15, 48, 87, 138, 342, 675, 901, 1323, 1362, 1485, 1909, 9498, 10941, 11563, 16308, 37495, 43875, 71166, 82960 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also, numbers k such that (7*10^k - 457)/9 is prime. Terms from Kamada. a(24) > 2*10^5. LINKS Makoto Kamada, Near-repdigit numbers of the form AA...AABA. Makoto Kamada, Prime numbers of the form 77...7727. EXAMPLE For k=4, 7*R_4 - 50 = 7777 - 50 = 7727 which is prime. MATHEMATICA Select[Range[0, 250000], PrimeQ[(7*10^# - 457)/9] &] PROG (MAGMA) [n: n in [2..400] | IsPrime((7*10^n - 457) div 9)]; // Vincenzo Librandi, Apr 11 2015 CROSSREFS Cf. A002275. Sequence in context: A192112 A034885 A280991 * A130324 A020677 A310007 Adjacent sequences:  A256723 A256724 A256725 * A256727 A256728 A256729 KEYWORD more,hard,nonn AUTHOR Robert Price, Apr 10 2015 EXTENSIONS a(21)-a(23) from Robert Price, Sep 28 2017 STATUS approved

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Last modified September 28 08:36 EDT 2021. Contains 347713 sequences. (Running on oeis4.)