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A256925
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Numbers k such that R_(k+2) + 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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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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Also, numbers k such that (109*10^k - 1)/9 is prime.
Terms from Kamada.
a(6) > 250000.
Numbers k such that 12 followed by k ones is prime. - Harvey P. Dale, Jan 30 2022
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LINKS
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MATHEMATICA
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Select[Range[0, 250000], PrimeQ[(109*10^#-1)/9 ] &]
Select[Range[700], PrimeQ[FromDigits[PadRight[{1, 2}, #, 1]]]&]-2 (* The program generates the first 3 terms of the sequence. To generate more, increase the Range constant but the program may take a long time to run. *) (* Harvey P. Dale, Jan 30 2022 *)
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PROG
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(Magma) [n: n in [0..300] | IsPrime((109*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 700, if(isprime((109*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
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CROSSREFS
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KEYWORD
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more,hard,nonn
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AUTHOR
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STATUS
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approved
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