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 A282457 Numbers k such that 7*10^k - 47 is prime. 0
 1, 2, 5, 8, 11, 14, 15, 30, 31, 42, 69, 85, 120, 128, 449, 685, 686, 741, 2165, 9110, 9150, 11366, 13154, 1291, 1521, 33321, 40758, 50789, 51440, 65492, 82509, 93887 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 53 is prime (see Example section). a(33) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 69w53. EXAMPLE 2 is in this sequence because 7*10^2 - 47 = 653 is prime. Initial terms and primes associated: a(1) = 1, 23; a(2) = 2, 653; a(3) = 5, 699953; a(4) = 8, 699999953; a(5) = 11, 699999999953; etc. MATHEMATICA Select[Range[1, 100000], PrimeQ[7*10^# - 47] &] CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A294391 A257771 A276881 * A329848 A059560 A022842 Adjacent sequences:  A282454 A282455 A282456 * A282458 A282459 A282460 KEYWORD nonn,more,hard AUTHOR Robert Price, Feb 15 2017 STATUS approved

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Last modified February 27 06:36 EST 2020. Contains 332299 sequences. (Running on oeis4.)