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 A256762 Numbers k such that R_k + 80 is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
 3, 6, 27, 30, 108, 119, 209, 444, 1050, 15189, 16401, 81363, 134385, 135011 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also, numbers k such that (10^k + 719)/9 is prime. Terms from Kamada data. a(15) > 2.5*10^5. LINKS Makoto Kamada, Near-repdigit numbers of the form AA...AABA. Makoto Kamada, Prime numbers of the form 11...1191. EXAMPLE For k=3, R_3 + 80 = 111 + 80 = 191 which is prime. MATHEMATICA Select[Range[0, 250000], PrimeQ[(10^# + 719)/9] &] PROG (MAGMA) [n: n in [2..300] | IsPrime((10^n+719) div 9)]; // Vincenzo Librandi, Apr 10 2015 CROSSREFS Cf. A002275. Sequence in context: A128437 A200654 A208665 * A064283 A266857 A014561 Adjacent sequences:  A256759 A256760 A256761 * A256763 A256764 A256765 KEYWORD nonn,more,hard AUTHOR Robert Price, Apr 09 2015 STATUS approved

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Last modified February 23 09:04 EST 2019. Contains 320420 sequences. (Running on oeis4.)