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A281512
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Numbers k such that 5*10^k - 81 is prime.
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0
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2, 3, 4, 13, 15, 22, 24, 28, 111, 148, 248, 294, 476, 1017, 1875, 2061, 2068, 2593, 4462, 6078, 14457, 14527, 23679, 33356, 36484, 50890, 51608, 113224
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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 4 followed by k-2 occurrences of the digit 9 followed by the digits 19 is prime (see Example section).
a(29) > 2*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because 5*10^3 - 81 = 4919 is prime.
Initial terms and primes associated:
a(1) = 2, 419;
a(2) = 3, 4919;
a(3) = 4, 49919;
a(4) = 13, 49999999999919;
a(5) = 15, 4999999999999919; etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[5*10^# - 81] &]
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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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