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%I #12 Sep 08 2022 08:46:13
%S 2,8,140,230,426,462,726,1974,7230,45860,47304
%N Numbers k such that 5*R_k + 3*10^k - 2 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
%C Also, numbers k such that (32*10^k - 23)/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/3/35553.htm#prime">Prime numbers of the form 355...553</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=2, 5*R_2 + 3*10^k - 2 = 55 + 300 - 2 = 353 which is prime.
%t Select[Range[0, 100000], PrimeQ[(32*10^#-23)/9] &]
%o (Magma) [n: n in [0..450] | IsPrime((32*10^n-23) div 9)]; // _Vincenzo Librandi_, Jun 19 2015
%Y Cf. A002275.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Jun 18 2015