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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.
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%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