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A287680
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Numbers k such that 8*10^k + 81 is prime.
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0
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0, 2, 3, 9, 11, 26, 39, 42, 58, 75, 105, 167, 279, 561, 2152, 2378, 3774, 5001, 5736, 9470, 13419, 14535, 32291, 89800, 146421
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OFFSET
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1,2
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COMMENTS
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For k>1, numbers such that the digit 8 followed by k-2 occurrences of the digit 0 followed by the digits 81 is prime (see Example section).
a(26) > 2*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because 8*10^3 + 81 = 8081 is prime.
Initial terms and primes associated:
a(1) = 0, 89;
a(2) = 2, 881;
a(3) = 3, 8081;
a(4) = 9, 8000000081;
a(5) = 11, 800000000081; etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(8*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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