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A282505
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Numbers k such that (5*10^k - 29)/3 is prime.
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0
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1, 2, 3, 4, 5, 6, 12, 32, 63, 116, 154, 221, 267, 468, 605, 749, 1911, 7241, 7406, 7797, 9428, 11094, 43917, 44127, 58384, 131223, 181127
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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 1 followed by k-2 occurrences of the digit 6 followed by the digits 57 is prime (see Example section).
a(28) > 2*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because (5*10^3 - 29)/3 = 1657 is prime.
Initial terms and associated primes:
a(1) = 1, 7;
a(2) = 2, 157;
a(3) = 3, 1657;
a(4) = 4, 16657;
a(5) = 5, 166657; etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[(5*10^# - 29)/3] &]
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PROG
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(PARI) isok(k) = isprime((5*10^k - 29)/3); \\ Michel Marcus, Mar 06 2018
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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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