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%I #13 Jan 17 2019 13:44:07
%S 1,3,4,9,12,60,268,289,384,795,3474,9180,76438
%N Numbers n such that 5*10^n + 4*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (49*10^n-13)/9 is prime.
%C a(14) > 10^5. - _Robert Price_, Jul 16 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/54443.htm#prime">Prime numbers of the form 544...443</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101579(n) + 1.
%t Do[ If[ PrimeQ[(49*10^n - 13)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101579.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 18 2005
%E a(13) from _Robert Price_, Jul 16 2015