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A259132
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Numbers k such that 5*R_k + 7*10^k + 2 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, 10, 20, 22, 58, 74, 82, 208, 350, 422, 3812, 3982, 20924, 23786, 38852, 56042, 68504, 74434
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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 (68*10^k + 13)/9 is prime.
Terms from Kamada data.
a(20) > 10^5.
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LINKS
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EXAMPLE
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For k=2, 5*R_2 + 7*10^k + 2 = 55 + 700 + 2 = 757 which is prime.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(68*10^#+13)/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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STATUS
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approved
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