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 A257039 Numbers k such that 9*R_(k+2) - 4*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
 10, 12, 63, 69, 156, 328, 340, 344, 444, 672, 894, 1464, 1670, 1708, 2010, 4306, 7888, 8864, 9478, 9621, 26004, 36992, 71600, 98738, 118949, 130565, 140326, 183452, 211983, 225618 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also, numbers k such that 96*10^n-1 is prime. Terms from Kamada. 846519 reported to Kamada by Bruno DallOsto is also in this sequence.  It may or may not be a(31). a(31) > 230000. LINKS Makoto Kamada, Near-repdigit numbers of the form ABAA...AA. Makoto Kamada, Prime numbers of the form 9599...99. EXAMPLE For k=10, 9*R_12 - 4*10^10 = 999999999999 - 40000000000 = 959999999999 which is prime. MATHEMATICA Select[Range[0, 230000], PrimeQ[96*10^#-1 ] &] PROG (MAGMA) [n: n in [0..400] | IsPrime(96*10^n-1)]; // Vincenzo Librandi, Apr 15 2015 CROSSREFS Cf. A002275. Sequence in context: A266700 A242508 A219917 * A223150 A333258 A241252 Adjacent sequences:  A257036 A257037 A257038 * A257040 A257041 A257042 KEYWORD more,hard,nonn AUTHOR Robert Price, Apr 14 2015 STATUS approved

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Last modified April 6 12:19 EDT 2020. Contains 333273 sequences. (Running on oeis4.)