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%I #16 Sep 08 2022 08:46:11
%S 2,3,4,7,9,37,39,67,73,183,216,1161,2622,2775,6240,22078,30384,189385
%N Numbers k such that 3*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 - 61)/3 is prime.
%C Terms from Kamada data.
%C a(19) > 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/3/33313.htm#prime">Prime numbers of the form 33...3313</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=3, 3*R_3 - 20 = 333 - 20 = 313 which is prime.
%t Select[Range[0, 250000], PrimeQ[(10^# - 61)/3] &]
%o (Magma) [n: n in [2..300] | IsPrime((10^n-61) div 3)]; // _Vincenzo Librandi_, Apr 10 2015
%Y Cf. A002275.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Apr 09 2015
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