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%I #18 Sep 08 2022 08:46:11
%S 1,2,8,56,675470,718580
%N Numbers k such that R_k + 30 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that (10^k + 269)/9 is prime.
%C Terms from Kamada data. Note that Kamada does not recognize k=1 as 31 is a degenerate case of form AAA..ABA.
%C a(7) > 10^6.
%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/1/11141.htm#prime">Prime numbers of the form 11...1141</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=8, R_8 + 30 = 11111111 + 30 = 11111141 which is prime.
%t Select[Range[0, 250000], PrimeQ[(10^# + 269)/9] &]
%o (Magma) [n: n in [1..400] | IsPrime((10^n+269) div 9)]; // _Vincenzo Librandi_, Apr 10 2015
%Y Cf. A002275.
%K more,hard,nonn
%O 1,2
%A _Robert Price_, Apr 09 2015
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