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%I #12 Jul 08 2021 01:19:57
%S 1,6,48,102,192,366,1002,20364,37446,56082
%N Numbers k such that 7*R_k + 10^k - 6 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
%C Also, numbers k such that (16*10^k - 61)/9 is prime.
%C Terms from Kamada data.
%C a(11) > 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/1/17771.htm#prime">Prime numbers of the form 177...771</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=6, 7*R_6 + 10^k - 6 = 777777 + 10000000 - 6 = 1777771 which is prime.
%t Select[Range[100000], PrimeQ[(16*10^#-61)/9] &]
%Y Cf. A002275.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Jun 18 2015
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