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 A256772 Numbers k such that 3*R_k - 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
 2, 3, 4, 7, 9, 37, 39, 67, 73, 183, 216, 1161, 2622, 2775, 6240, 22078, 30384, 189385 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also, numbers k such that (10^k - 61)/3 is prime. Terms from Kamada data. a(19) > 2.5*10^5. LINKS Table of n, a(n) for n=1..18. Makoto Kamada, Near-repdigit numbers of the form AA...AABA. Makoto Kamada, Prime numbers of the form 33...3313. Index entries for primes involving repunits. EXAMPLE For k=3, 3*R_3 - 20 = 333 - 20 = 313 which is prime. MATHEMATICA Select[Range[0, 250000], PrimeQ[(10^# - 61)/3] &] PROG (Magma) [n: n in [2..300] | IsPrime((10^n-61) div 3)]; // Vincenzo Librandi, Apr 10 2015 CROSSREFS Cf. A002275. Sequence in context: A319645 A071114 A078696 * A274976 A269729 A098115 Adjacent sequences: A256769 A256770 A256771 * A256773 A256774 A256775 KEYWORD nonn,more,hard AUTHOR Robert Price, Apr 09 2015 STATUS approved

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Last modified June 15 16:27 EDT 2024. Contains 373407 sequences. (Running on oeis4.)