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A259135
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Numbers k such that 2*R_k + 7*10^k + 5 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
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0
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2, 4, 8, 28, 64, 724, 1786, 7276, 19462, 24214, 51778, 131392
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OFFSET
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1,1
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COMMENTS
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Also, numbers k such that (65*10^k + 43)/9 is prime.
Terms from Kamada data.
a(12) > 10^5.
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LINKS
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EXAMPLE
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For k=2, 2*R_2 + 7*10^k + 5 = 22 + 700 + 5 = 727 which is prime.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(65*10^#+43)/9] &]
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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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