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%I #14 Sep 08 2022 08:46:11
%S 2,3,5,8,9,11,18,20,29,39,123,153,267,327,627,1020,1686,1764,2960,
%T 18236,34613,53991,67404,119880,221654,230706
%N Numbers k such that R_k + 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that (10^k + 179)/9 is prime.
%C Terms from Kamada data.
%C a(27) > 2.5*10^5.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/aaaba.htm">Near-repdigit numbers of the form AA...AABA</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11131.htm#prime">Prime numbers of the form 11...1131</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=3, R_3 + 20 = 111 + 20 = 131 which is prime.
%t Select[Range[0, 250000], PrimeQ[(10^# + 179)/9] &]
%o (Magma) [n: n in [1..400] | IsPrime((10^n+179) div 9)]; // _Vincenzo Librandi_, Apr 10 2015
%Y Cf. A002275.
%K more,hard,nonn
%O 1,1
%A _Robert Price_, Apr 09 2015
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