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%I #12 Jan 17 2019 13:44:07
%S 2,3,6,12,36,39,44,90,258,512,1482,2378,2406,3224,3378,3876,4161,5510,
%T 7425,97242
%N Numbers n such that 3*10^n + 4*R_n + 5 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (31*10^n+41)/9 is prime.
%C a(21) > 10^5. - _Robert Price_, Apr 04 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/34449.htm#prime">Prime numbers of the form 344...449</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101834(n) + 1.
%t Do[ If[ PrimeQ[(31*10^n+41)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101834.
%K more,nonn
%O 1,1
%A _Robert G. Wilson v_, Dec 17 2004
%E a(20) from _Robert Price_, Apr 04 2015
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