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%I #25 Sep 08 2022 08:45:16
%S 0,1,4,18,19,646,814,5794,16524,19495,26010,29238,72120
%N Numbers n such that 7*10^n + 5*R_n - 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (68*10^n-23)/9 is prime.
%C a(14) > 10^5. - _Robert Price_, Oct 01 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/75553.htm#prime">Prime numbers of the form 755...553</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101144(n-1) + 1 for n>1.
%t Do[ If[ PrimeQ[(68*10^n - 23)/9], Print[n]], {n, 0, 10000}]
%t Select[Range[10000], PrimeQ[(68 10^# - 23)/9] &] (* _Vincenzo Librandi_, Oct 02 2015 *)
%o (Magma) [n: n in [0..100]| IsPrime((68*10^n-23) div 9)]; // _Vincenzo Librandi_, Oct 02 2015
%o (PARI) for(n=0, 1e4, if (isprime((68*10^n-23)/9), print1(n", "))) \\ _Altug Alkan_, Oct 02 2015
%Y Cf. A002275, A101144.
%K more,nonn
%O 1,3
%A _Robert G. Wilson v_, Jan 19 2005
%E a(9)-a(12) from Kamada data by _Robert Price_, Dec 14 2010
%E a(1)=0 inserted and a(13) added by _Robert Price_, Oct 01 2015