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A098089
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Numbers k such that 7*R_k + 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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3
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0, 2, 66, 86, 90, 102, 386, 624, 7784, 18536, 113757, 135879
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OFFSET
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1,2
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COMMENTS
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Also numbers k such that (7*10^k + 11)/9 is prime.
Although perhaps a degenerate case, A002275 defines R(0)=0. Thus zero belongs in this sequence since 7*0 + 2 = 2 is prime. - Robert Price, Oct 28 2014
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LINKS
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FORMULA
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EXAMPLE
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If n = 2, we get ((7*10^2)+11/9 = (700+11)/9 = 79, which is prime.
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MATHEMATICA
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Do[ If[ PrimeQ[ 7(10^n - 1)/9 + 2], Print[n]], {n, 0, 5000}] (* Robert G. Wilson v, Oct 15 2004 *)
Do[ If[ PrimeQ[((7*10^n) + 11)/9], Print[n]], {n, 0, 8131}] (* Robert G. Wilson v, Sep 27 2004 *)
Select[Range[0, 700], PrimeQ[(7 10^# + 11) / 9] &] (* Vincenzo Librandi, Nov 22 2014 *)
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PROG
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(Magma) [n: n in [0..300] | IsPrime((7*10^n+11) div 9)]; // Vincenzo Librandi, Nov 22 2014
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CROSSREFS
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KEYWORD
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more,nonn
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AUTHOR
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Julien Peter Benney (jpbenney(AT)ftml.net), Sep 14 2004
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EXTENSIONS
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a(1)=0 added and Mathematica programs adapted by Robert Price, Oct 28 2014
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STATUS
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approved
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