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A293911
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Numbers k such that (86*10^k - 311)/9 is prime.
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0
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1, 3, 6, 13, 31, 58, 76, 96, 147, 174, 306, 532, 1177, 3019, 5790, 5952, 7489, 10269, 10936, 20209, 28188, 55645, 71068, 89640
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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 9 followed by k-2 occurrences of the digit 5 followed by the digits 21 is prime (see Example section).
a(25) > 2*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because (86*10^3 - 311)/9 = 9521 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 3, 9521;
a(3) = 6, 9555521;
a(4) = 13, 95555555555521;
a(5) = 31, 95555555555555555555555555555521; etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[(86*10^# - 311)/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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STATUS
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approved
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