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A294939
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Numbers k such that (8*10^k + 13)/3 is prime.
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0
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0, 1, 2, 3, 5, 8, 13, 69, 80, 344, 405, 500, 794, 926, 3293, 3964, 7203, 7395, 8163, 14433, 68455, 108273, 137845
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OFFSET
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1,3
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COMMENTS
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For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 6 followed by the digits 71 is prime (see Example section).
a(24) > 2*10^5.
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LINKS
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EXAMPLE
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2 is in this sequence because (8*10^2 + 13)/3 = 271 is prime.
Initial terms and associated primes:
a(1) = 0, 7;
a(2) = 1, 31;
a(3) = 2, 271;
a(4) = 3, 2671;
a(5) = 5, 266671; etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(8*10^# + 13)/3] &]
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PROG
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(PARI) isok(k) = isprime((8*10^k + 13)/3); \\ Michel Marcus, Nov 12 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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