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%I #22 Jul 08 2021 05:10:18
%S 2,5,6,8,17,50,684,720,1452,1679,3146,72821
%N Numbers k such that R_k + 8 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (10^k + 71)/9 is prime.
%C The corresponding values R_k + 8 are primes of the form "(k-1) ones followed by a nine". Related to the base-10 repunit primes.
%C a(13) > 10^5. - _Robert Price_, Nov 01 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11119.htm#prime">Prime numbers of the form 11...119</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A056659(n) + 1.
%t Do[ If[ PrimeQ[(10^n - 1)/9 + 8], Print[n]], {n, 0, 5000}] (* _Robert G. Wilson v_, Oct 14 2004 *)
%Y Cf. A004023, A056659, A097683, A097684.
%K nonn,more,hard
%O 1,1
%A _Carl R. White_, Aug 19 2004
%E a(12) from _Robert Price_, Nov 01 2014
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