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A286936
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Numbers k such that (71*10^k - 287)/9 is prime.
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0
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1, 2, 4, 8, 26, 28, 43, 70, 92, 128, 331, 364, 478, 532, 689, 778, 895, 1210, 5081, 7855, 17852, 20864, 42598, 56858, 120703, 173854
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OFFSET
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1,2
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COMMENTS
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For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 8 followed by the digits 57 is prime (see Example section).
a(27) > 2*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because (71*10^4 - 287)/9 = 78857 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 2, 757;
a(3) = 4, 78857;
a(4) = 8, 788888857;
a(5) = 26, 788888888888888888888888857; etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[(71*10^# - 287)/9] &]
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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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