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%I #30 Sep 08 2022 08:46:11
%S 3,4,7,12,15,48,87,138,342,675,901,1323,1362,1485,1909,9498,10941,
%T 11563,16308,37495,43875,71166,82960
%N Numbers k such that 7*R_k - 50 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that (7*10^k - 457)/9 is prime.
%C Terms from Kamada.
%C a(24) > 2*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/7/77727.htm#prime">Prime numbers of the form 77...7727</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=4, 7*R_4 - 50 = 7777 - 50 = 7727 which is prime.
%t Select[Range[0, 250000], PrimeQ[(7*10^# - 457)/9] &]
%o (Magma) [n: n in [2..400] | IsPrime((7*10^n - 457) div 9)]; // _Vincenzo Librandi_, Apr 11 2015
%Y Cf. A002275.
%K more,hard,nonn
%O 1,1
%A _Robert Price_, Apr 10 2015
%E a(21)-a(23) from _Robert Price_, Sep 28 2017
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