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%I #12 Jul 08 2021 00:45:26
%S 1,2,8,14,40,92,128,884,9424,14768,19258,31234
%N Numbers k such that 8*R_k + 10^k - 7 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
%C Also, numbers k such that (17*10^k - 71)/9 is prime.
%C Terms from Kamada data.
%C a(13) > 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/1/18881.htm#prime">Prime numbers of the form 188...881</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=8, 8*R_8 + 10^k - 7 = 88888888 + 100000000 - 7 = 188888881 which is prime.
%t Select[Range[0, 100000], PrimeQ[(17*10^#-71)/9] &]
%Y Cf. A002275.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Jun 18 2015
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