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 A257041 Numbers k such that 9*R_(k+2) - 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
 3, 4, 5, 9, 19, 25, 34, 70, 82, 718, 2035, 2494, 3369, 3953, 4219, 8524, 30943, 49375, 52082, 139670, 303255 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also, numbers k such that 99*10^k - 1 is prime. Terms from Kamada. LINKS Table of n, a(n) for n=1..21. Makoto Kamada, Near-repdigit numbers of the form ABAA...AA. Makoto Kamada, Prime numbers of the form 9899...99. Index entries for primes involving repunits. EXAMPLE For k=3, 9*R_5 - 10^3 = 99999 - 1000 = 98999 which is prime. MATHEMATICA Select[Range[0, 280000], PrimeQ[99*10^#-1 ] &] PROG (Magma) [n: n in [0..300] | IsPrime(99*10^n-1)]; // Vincenzo Librandi, Apr 15 2015 CROSSREFS Cf. A002275. Sequence in context: A080552 A215176 A063781 * A346603 A056797 A029727 Adjacent sequences: A257038 A257039 A257040 * A257042 A257043 A257044 KEYWORD more,hard,nonn AUTHOR Robert Price, Apr 14 2015 EXTENSIONS a(21) from Predrag Kurtovic, May 24 2015 STATUS approved

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Last modified August 9 10:32 EDT 2024. Contains 375040 sequences. (Running on oeis4.)