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A293757
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Numbers k such that (10^k - 97)/3 is prime.
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0
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4, 5, 7, 13, 16, 22, 30, 56, 78, 80, 90, 194, 316, 1796, 2004, 3856, 5226, 6737, 10841, 43638, 50467, 85666
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OFFSET
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1,1
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COMMENTS
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For k > 1, numbers k such that k-2 occurrences of the digit 3 followed by the digits 01 is prime (see Example section).
a(23) > 2*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because (10^4 - 97)/3 = 3301 is prime.
Initial terms and associated primes:
a(1) = 4, 3301;
a(2) = 5, 33301;
a(3) = 7, 3333301;
a(4) = 13, 3333333333301;
a(5) = 16, 3333333333333301; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[(10^# - 97)/3] &]
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PROG
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(Magma) [n: n in [1..400] |IsPrime((10^n-97) div 3)]; // Vincenzo Librandi, Oct 16 2017
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CROSSREFS
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KEYWORD
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nonn,more,hard
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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