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%I #22 Jun 19 2020 11:49:29
%S 3,9,15,21,27,87,180,1941,3960,4995,8577,23481,27855,80595
%N Numbers n such that 2*10^n + 4*R_n + 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (22*10^n+23)/9 is prime.
%C a(15) > 10^5. - _Robert Price_, Mar 15 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/24447.htm#prime">Prime numbers of the form 244...447</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101959(n) + 1.
%t Do[ If[ PrimeQ[(22*10^n + 23)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101959.
%K more,nonn
%O 1,1
%A _Robert G. Wilson v_, Dec 17 2004
%E Addition of a(12)-a(13) from Kamada data by _Robert Price_, Dec 12 2010
%E a(14) from _Robert Price_, Mar 15 2015
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