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A295082
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Numbers k such that (17*10^k - 521)/9 is prime.
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0
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2, 3, 5, 8, 9, 14, 20, 21, 29, 41, 113, 677, 3011, 3015, 3083, 3140, 4473, 4947, 138812
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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 1 followed by k-2 occurrences of the digit 8 followed by the digits 31 is prime (see Example section).
a(20) > 2*10^5.
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LINKS
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EXAMPLE
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2 is in this sequence because (17*10^2 - 521)/9 = 131 is prime.
Initial terms and associated primes:
a(1) = 2, 131;
a(2) = 3, 1831;
a(3) = 5, 188831;
a(4) = 8, 188888831;
a(5) = 9, 1888888831; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[(17*10^# - 521)/9] &]
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PROG
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(PARI) isok(k) = isprime((17*10^k - 521)/9); \\ Michel Marcus, Nov 14 2017
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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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