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A259136
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Numbers k such that 4*R_k + 7*10^k + 3 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
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0
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10, 30, 120, 484, 1486, 1578, 13672, 13810, 15094, 72772, 94212
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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 (67*10^k + 23)/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=10, 4*R_2 + 7*10^k + 3 = 4444444444 + 7000000000 + 3 = 74444444447 which is prime.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(67*10^#+23)/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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