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%I #17 Jul 08 2021 03:08:55
%S 1,3,6,15,70,76,13024,60346
%N Numbers k such that 10^k + 5*R_k - 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (14*10^k - 23)/9 is prime.
%C a(9) > 10^5. - _Robert Price_, Apr 10 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/15553.htm#prime">Prime numbers of the form 155...553</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102019(n) + 1.
%t Do[ If[ PrimeQ[(14*10^n - 23)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A102019.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Dec 16 2004
%E Addition of a(7) from Kamada data by _Robert Price_, Dec 13 2010
%E a(8) from _Robert Price_, Apr 10 2015
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