%I #17 Jan 17 2019 13:44:07
%S 0,5,6,185,401,480,1319,5646,6029,16824,62387,70638
%N Numbers n such that 8*10^n + 7*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (79*10^n-61)/9 is prime.
%C a(13) > 10^5. - _Robert Price_, Oct 25 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/87771.htm#prime">Prime numbers of the form 877...771</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101076(n+1) + 1, for n>1.
%t Do[ If[ PrimeQ[(79*10^n - 61)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101076.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 19 2005
%E 1 more term. Next term exceeds 50000. _Sean A. Irvine_, Sep 10 2009
%E Inserted a(1)=0 and added a(11)-a(12) by _Robert Price_, Oct 25 2015