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%I #14 Feb 01 2023 18:07:21
%S 2,11,24,42,56,336,738,2712,3498,8984,14036,46439
%N Numbers k such that 7*R_k - 10 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that (7*10^k - 97)/9 is prime.
%C Terms from Kamada.
%C a(12) > 30000.
%C a(13) > 2*10^5. - _Tyler Busby_, Feb 01 2023
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/aaaba.htm">Near-repdigit numbers of the form AA...AABA</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77767.htm#prime">Prime numbers of the form 77...7767</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=11, 7*R_11 - 10 = 77777777777 - 10 = 77777777767 which is prime.
%t Select[Range[0, 30000], PrimeQ[(7*10^# - 97)/9] &]
%Y Cf. A002275.
%K more,hard,nonn
%O 1,1
%A _Robert Price_, Apr 12 2015
%E a(12) from _Tyler Busby_, Feb 01 2023