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A295969
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Numbers k such that (5*10^k - 473)/9 is prime.
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0
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2, 3, 4, 9, 10, 16, 27, 46, 97, 103, 214, 229, 241, 285, 643, 999, 3154, 14115, 43012, 85855, 87622, 87712, 90142, 168379
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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 k-2 occurrences of the digit 5 followed by the digits 03 is prime (see Example section).
a(25) > 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 - 473)/9 = 503 is prime.
Initial terms and associated primes:
a(1) = 2, 3;
a(2) = 3, 503;
a(3) = 4, 5503;
a(4) = 9, 555555503;
a(5) = 10, 5555555503; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[(5*10^# - 473)/9] &]
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PROG
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(PARI) isok(k) = isprime((5*10^k - 473)/9); \\ Michel Marcus, Dec 01 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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