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A287017
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Positive numbers k such that (10^(k+2)*109 + 89)/9 is prime.
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0
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OFFSET
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1,1
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COMMENTS
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Or '12'||...'1'...||'21' in decimal form is prime ('1' concatenated k times to which the prefix '12' and the suffix '21' are concatenated once).
a(1)...a(6) themselves are primes. a(7) > 70000 (if it exists).
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LINKS
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EXAMPLE
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5 is a term as 121111121 is prime (as a string, it consists of '1' concatenated 5 times to which the prefix '12' and the suffix '21' are concatenated once).
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MATHEMATICA
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ParallelMap[ If[ PrimeQ[ (10^(#+2)*109+89)/9], #, Nothing]&, Range[3500]]
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PROG
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CROSSREFS
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KEYWORD
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nonn,hard,more,base
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AUTHOR
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STATUS
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approved
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