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%I #12 Jul 08 2021 00:45:23
%S 1,2,4,8,40,86,200,730,1460,23672,28630
%N Numbers k such that 9*R_k + 10^k - 8 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
%C Also, numbers k such that 2*10^k - 9 is prime.
%C Terms from Kamada data.
%C a(12) > 2*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/1/19991.htm#prime">Prime numbers of the form 199...991</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=4, 9*R_4 + 10^k - 2 = 9999 + 10000 - 8 = 19991 which is prime.
%t Select[Range[200000], PrimeQ[2*10^n-9] &]
%Y Cf. A002275.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Jun 18 2015
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