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 A256828 Numbers k such that 7*R_k - 40 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
 2, 6, 18, 48, 204, 10386, 19524, 28320 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also, numbers k such that (7*10^k - 367)/9 is prime. Terms from Kamada. a(9) > 30000. a(9) > 2*10^5. - Tyler Busby, Feb 02 2023 LINKS Table of n, a(n) for n=1..8. Makoto Kamada, Near-repdigit numbers of the form AA...AABA. Makoto Kamada, Prime numbers of the form 77...7737. Index entries for primes involving repunits. EXAMPLE For k=6, 7*R_6 - 40 = 777777 - 40 = 777737 which is prime. MATHEMATICA Select[Range[0, 250000], PrimeQ[(7*10^# - 367)/9] &] PROG (Magma) [n: n in [2..300] | IsPrime((7*10^n-367) div 9)]; // Vincenzo Librandi, Apr 11 2015 CROSSREFS Cf. A002275. Sequence in context: A218759 A295499 A059413 * A197055 A258625 A062026 Adjacent sequences: A256825 A256826 A256827 * A256829 A256830 A256831 KEYWORD more,hard,nonn AUTHOR Robert Price, Apr 10 2015 STATUS approved

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Last modified August 9 11:22 EDT 2024. Contains 375042 sequences. (Running on oeis4.)