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%I #25 Jul 08 2021 03:09:02
%S 0,1,2,5,7,19,20,28,58,250,397,7591,32716,70978,83587
%N Numbers k such that 10^k + 5*R_k + 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (14*10^k + 13)/9 is prime.
%C a(16) > 10^5. - _Robert Price_, Jan 17 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/15557.htm#prime">Prime numbers of the form 155...557</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102020(n-1) + 1.
%t Do[ If[ PrimeQ[(14*10^n + 13)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A102020.
%K more,nonn
%O 1,3
%A _Robert G. Wilson v_, Dec 16 2004
%E 0 added by _Arkadiusz Wesolowski_, Mar 10 2011
%E a(13) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
%E a(14)-a(15) from _Robert Price_, Jan 17 2015
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