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%I #10 Jul 08 2021 01:30:38
%S 2,4,8,28,64,724,1786,7276,19462,24214,51778
%N Numbers k such that 2*R_k + 7*10^k + 5 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
%C Also, numbers k such that (65*10^k + 43)/9 is prime.
%C Terms from Kamada data.
%C a(12) > 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/7/72227.htm#prime">Prime numbers of the form 72...27</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=2, 2*R_2 + 7*10^k + 5 = 22 + 700 + 5 = 727 which is prime.
%t Select[Range[0, 100000], PrimeQ[(65*10^#+43)/9] &]
%Y Cf. A002275.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Jun 18 2015
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