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A289752
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Numbers k such that (4*10^k - 79)/3 is prime.
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0
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2, 3, 8, 11, 20, 24, 32, 33, 60, 86, 162, 410, 816, 1809, 1920, 2499, 2922, 7205, 8238, 15990, 37163, 115668, 141926, 179963
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OFFSET
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1,1
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COMMENTS
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For k>1, numbers such that the digit 1 followed by k-2 occurrences of the digit 3 followed by the digits 07 is prime (see Example section).
a(25) > 2*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because (4*10^3 - 79)/3 = 1307 is prime.
Initial terms and primes associated:
a(1) = 2, 107;
a(2) = 3, 1307;
a(3) = 8, 133333307;
a(4) = 11, 133333333307;
a(5) = 20, 133333333333333333307; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[(4*10^# - 79)/3] &]
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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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