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A295324
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Numbers k such that (17*10^k - 89)/9 is prime.
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0
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2, 3, 6, 12, 39, 45, 50, 75, 126, 282, 431, 459, 1557, 2724, 3453, 4058, 15879, 24209, 31844, 61760, 155133
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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 79 is prime (see Example section).
a(22) > 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 - 89)/9 = 179 is prime.
Initial terms and primes associated:
a(1) = 2, 179;
a(2) = 3, 1879;
a(3) = 6, 1888879;
a(4) = 12, 1888888888879;
a(5) = 39, 1888888888888888888888888888888888888879; etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[(17*10^# - 89)/9] &]
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PROG
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(PARI) isok(k) = isprime((17*10^k - 89)/9); \\ Michel Marcus, Nov 20 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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