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%I #15 Sep 08 2022 08:46:13
%S 2,14,54,68,84,86,156,2766,3380,3876,5208,10746,15768,31316,40958,
%T 45804,46566,51008,80162
%N Numbers k such that 7*R_k + 3*10^k - 4 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
%C Also, numbers k such that (34*10^k - 43)/9 is prime.
%C Terms from Kamada data.
%C a(20) > 10^5.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/abbba.htm">Near-repdigit numbers of the form ABB...BBA</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/37773.htm#prime">Prime numbers of the form 377...773</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=2, 7*R_2 + 3*10^k - 4 = 77 + 300 - 4 = 373 which is prime.
%t Select[Range[0, 100000], PrimeQ[(34*10^#-43)/9] &]
%o (Magma) [n: n in [0..450] | IsPrime((34*10^n-43) div 9)]; // _Vincenzo Librandi_, Jun 19 2015
%Y Cf. A002275.
%K nonn,hard,more
%O 1,1
%A _Robert Price_, Jun 18 2015
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