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A290115
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Numbers k such that (7*10^k + 197)/3 is prime.
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0
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1, 3, 4, 6, 7, 52, 53, 103, 131, 199, 294, 426, 780, 1144, 1876, 2001, 3507, 5737, 6657, 12558, 28303, 31608, 60643, 74741, 124648
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OFFSET
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1,2
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COMMENTS
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For k>1, numbers such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 99 is prime (see Example section).
a(26) > 2*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because (7*10^4 + 197)/3 = 23399 is prime.
Initial terms and primes associated:
a(1) = 1, 89;
a(2) = 3, 2399;
a(3) = 4, 23399;
a(4) = 6, 2333399;
a(5) = 7, 23333399; etc.
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MAPLE
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select(k -> isprime((7*10^k+197)/3), [$1..10000]); # Robert Israel, Jul 20 2017
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(7*10^# + 197)/3] &]
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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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