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 A256761 Numbers k such that R_k + 70 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
 1, 3, 4, 7, 9, 10, 16, 124, 243, 303, 459, 471, 520, 591, 1726, 2971, 5560, 5757, 13222 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, numbers k such that (10^k + 629)/9 is prime. Terms from Kamada data. Note that Kamada does not recognize k=1 as 81 is a degenerate case of form AAA..ABA. a(20) > 2.5*10^5. LINKS Table of n, a(n) for n=1..19. Makoto Kamada, Near-repdigit numbers of the form AA...AABA. Makoto Kamada, Prime numbers of the form 11...1181. Index entries for primes involving repunits. EXAMPLE For k=3, R_3 + 70 = 111 + 70 = 181 which is prime. MATHEMATICA Select[Range[0, 250000], PrimeQ[(10^# + 629)/9] &] PROG (Magma) [n: n in [1..350] | IsPrime((10^n+629) div 9)]; // Vincenzo Librandi, Apr 10 2015 CROSSREFS Cf. A002275. Sequence in context: A024799 A240531 A212013 * A039579 A274794 A115104 Adjacent sequences: A256758 A256759 A256760 * A256762 A256763 A256764 KEYWORD nonn,more,hard AUTHOR Robert Price, Apr 09 2015 STATUS approved

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Last modified July 14 12:35 EDT 2024. Contains 374318 sequences. (Running on oeis4.)