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A294917
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Numbers k such that 4*10^k - 71 is prime.
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0
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3, 4, 6, 11, 12, 17, 21, 54, 82, 112, 189, 235, 426, 442, 4897, 12463, 14383, 18630, 38669, 95752, 108238, 179265
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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 the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 29 is prime (see Example section).
a(23) > 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 - 71 = 3929 is prime.
Initial terms and associated primes:
a(1) = 3, 3929;
a(2) = 4, 39929;
a(3) = 6, 3999929;
a(4) = 11, 399999999929;
a(5) = 12, 3999999999929; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[4*10^# - 71] &]
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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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