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%I #19 Jan 17 2019 13:44:07
%S 1,4,12,15,18,37,109,1728,2482,3480,6577,9015,16162,45349
%N Numbers n such that 5*10^n + 7*R_n - 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (52*10^n-43)/9 is prime.
%C a(15) > 10^5. - _Robert Price_, Sep 09 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/57773.htm#prime">Prime numbers of the form 577...773</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101585(n) + 1.
%t Do[ If[ PrimeQ[(52*10^n - 43)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101585.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 18 2005
%E Addition of a(13) from Kamada data by _Robert Price_, Dec 13 2010
%E a(14) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
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