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A295399
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Numbers k such that (8*10^k - 611)/9 is prime.
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0
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3, 4, 7, 9, 48, 52, 105, 153, 1783, 1999, 2787, 4276, 13693, 14067, 19143, 20431, 25933, 26206, 29062, 40462, 40791, 58738, 81709
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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 k-2 occurrences of the digit 8 followed by the digits 21 is prime (see Example section).
a(24) > 2*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because (8*10^3 - 611)/9 = 821 is prime.
Initial terms and primes associated:
a(1) = 3, 821;
a(2) = 4, 8821;
a(3) = 7, 8888821;
a(4) = 9, 888888821; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[(8*10^# - 611)/9] &]
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PROG
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(PARI) isok(k) = isprime((8*10^k - 611)/9); \\ Michel Marcus, Nov 22 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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