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%I #20 Sep 08 2022 08:46:12
%S 2,5,23,182,209,287,7454,16958
%N Numbers k such that 7*R_(k+2) - 3*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that (673*10^k - 7)/9 is prime.
%C Terms from Kamada.
%C a(9) > 30000.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/abaaa.htm">Near-repdigit numbers of the form ABAA...AA</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/74777.htm#prime">Prime numbers of the form 7477...77</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=5, 7*R_7 - 3*10^5 = 7777777 - 300000 = 7477777 which is prime.
%t Select[Range[0, 30000], PrimeQ[(673*10^#-7)/9 ] &]
%o (Magma) [n: n in [0..400] | IsPrime((673*10^n-7) div 9)]; // _Vincenzo Librandi_, Apr 15 2015
%Y Cf. A002275.
%K more,hard,nonn
%O 1,1
%A _Robert Price_, Apr 14 2015
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