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A295967
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Numbers k such that (35*10^k - 377)/9 is prime.
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0
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2, 3, 6, 24, 33, 86, 335, 711, 1103, 1169, 1181, 2163, 3026, 7940, 8174, 9365, 10127, 13466, 40130, 70130, 174704, 198294
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OFFSET
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1,1
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COMMENTS
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For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 8 followed by the digits 47 is prime (see Example section).
a(23) > 2*10^5.
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LINKS
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EXAMPLE
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2 is in this sequence because (35*10^2 - 377)/9 = 347 is prime.
Initial terms and associated primes:
a(1) = 2, 347;
a(2) = 3, 3847;
a(3) = 6, 3888847;
a(4) = 24, 3888888888888888888888847;
a(5) = 33, 3888888888888888888888888888888847; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[(35*10^# - 377)/9] &]
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PROG
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(PARI) isok(k) = isprime((35*10^k - 377)/9); \\ Michel Marcus, Dec 01 2017
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CROSSREFS
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KEYWORD
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nonn,more,hard,changed
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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