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A282457
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Numbers k such that 7*10^k - 47 is prime.
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0
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1, 2, 5, 8, 11, 14, 15, 30, 31, 42, 69, 85, 120, 128, 449, 685, 686, 741, 1291, 1521, 2165, 9110, 9150, 11366, 13154, 33321, 40758, 50789, 51440, 65492, 82509, 93887
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OFFSET
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1,2
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COMMENTS
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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.
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LINKS
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Table of n, a(n) for n=1..32.
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 69w53.
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EXAMPLE
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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.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[7*10^# - 47] &]
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CROSSREFS
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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
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KEYWORD
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nonn,more,hard
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AUTHOR
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Robert Price, Feb 15 2017
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EXTENSIONS
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Reordered terms into ascending order by Robert Price, Apr 01 2022
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STATUS
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approved
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