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%I #18 Jan 17 2019 13:44:07
%S 21,27,60,471,1074,5781
%N Numbers n such that 8*10^n + 5*R_n - 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (77*10^n-41)/9 is prime.
%C a(7) > 10^5. - _Robert Price_, Oct 21 2015
%C It appears that all terms are divisible by 3. - _Robert Price_, Oct 21 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/85551.htm#prime">Prime numbers of the form 855...551</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101070(n) + 1.
%t Do[ If[ PrimeQ[(77*10^n - 41)/9], Print[n]], {n, 0, 10000}]
%o (PARI) for(n=1, 1e4, if(isprime((77*10^n-41)/9), print1(n, ", "))) \\ _Altug Alkan_, Oct 21 2015
%Y Cf. A002275, A101070.
%K more,nonn
%O 1,1
%A _Robert G. Wilson v_, Jan 19 2005
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