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%I #17 Feb 01 2023 17:37:15
%S 3,4,24,106,124,162,243,258,1344,1386,2494,4200,5859,8844,13122,19908,
%T 86844,106066,180732
%N Numbers k such that 7*R_k - 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that (7*10^k - 187)/9 is prime.
%C Terms from Kamada.
%C a(17) > 30000.
%C a(20) > 2*10^5. - _Tyler Busby_, Feb 01 2023
%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/7/77757.htm#prime">Prime numbers of the form 77...7757</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=4, 7*R_4 - 20 = 7777 - 20 = 7757 which is prime.
%t Select[Range[0, 250000], PrimeQ[(7*10^# - 187)/9] &]
%o (Magma) [n: n in [2..400] | IsPrime((7*10^n-187) 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(17)-a(19) from _Tyler Busby_, Feb 01 2023
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