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%I #25 Sep 08 2022 08:46:12
%S 0,1,3,9,16,30,45,123,171,295,324,2785,2791,3910,4015,4050,6319,6415,
%T 14670
%N Numbers k such that 7*R_(k+2) + 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that (718*10^k - 7)/9 is prime.
%C Terms from Kamada.
%C a(20) > 2*10^5.
%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/7/79777.htm#prime">Prime numbers of the form 7977...77</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=3, 7*R_5 + 2*10^3 = 77777 + 2000 = 79777 which is prime.
%t Select[Range[0, 30000], PrimeQ[(718*10^#-7)/9 ] &]
%o (Magma) [n: n in [0..400] | IsPrime((718*10^n-1) div 9)]; // _Vincenzo Librandi_, Apr 15 2015
%Y Cf. A002275.
%K nonn,hard,more
%O 1,3
%A _Robert Price_, Apr 14 2015
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