A093404
Primes of the form 70*R_k + 9, where R_k is the repunit (A002275) of length k.
Original entry on oeis.org
79, 777777777777777777777777777777777777777777777777777777777777777779, 77777777777777777777777777777777777777777777777777777777777777777777777777777777777779, 777777777777777777777777777777777777777777777777777777777777777777777777777777777777777779
Offset: 1
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Select[Table[FromDigits[PadLeft[{9},n,7]],{n,100}],PrimeQ] (* Harvey P. Dale, May 09 2012 *)
A098089
Numbers k such that 7*R_k + 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
Original entry on oeis.org
0, 2, 66, 86, 90, 102, 386, 624, 7784, 18536, 113757, 135879
Offset: 1
Julien Peter Benney (jpbenney(AT)ftml.net), Sep 14 2004
If k = 2, we get (7*10^2 + 11)/9 = (700+11)/9 = 79, which is prime.
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[n: n in [0..300] | IsPrime((7*10^n+11) div 9)]; // Vincenzo Librandi, Nov 22 2014
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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 *)
a(1)=0 added and Mathematica programs adapted by
Robert Price, Oct 28 2014
Showing 1-2 of 2 results.
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