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A293853
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Numbers k such that (10^k - 79)/3 is prime.
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0
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2, 3, 4, 7, 9, 15, 57, 63, 147, 178, 202, 697, 713, 952, 1861, 7433, 14311, 16737, 29369, 72723, 121543
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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 k - 2 occurrences of the digit 3 followed by the digits 07 is prime (see Example section).
a(22) > 2*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because (10^3 - 79)/3 = 307 is prime.
Initial terms and primes associated:
a(1) = 2, 7;
a(2) = 3, 307;
a(3) = 4, 3307;
a(4) = 7, 3333307;
a(5) = 9, 333333307; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[(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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