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%I #26 Sep 08 2022 08:45:16
%S 1,2,12,15,51,194,498,2136,2822,3762,7428,22740,30452,37952,55254
%N Numbers n such that 3*10^n + 6*R_n + 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (11*10^n+1)/3 is prime.
%C a(16) > 10^5. - _Robert Price_, Jan 29 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/36667.htm#prime">Prime numbers of the form 366...667</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101840(n) + 1.
%t Do[ If[ PrimeQ[(11*10^n+1)/3], Print[n]], {n, 0, 10000}]
%o (Magma) [n: n in [0..300] | IsPrime((11*10^n+1) div 3)]; // _Vincenzo Librandi_, Dec 01 2015
%Y Cf. A002275, A101840.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Dec 17 2004
%E Addition of a(12) from Kamada data by _Robert Price_, Dec 13 2010
%E a(13)-a(14) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
%E a(15) from Kamada data by _Robert Price_, Jan 29 2015
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