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%I #23 Jan 17 2019 13:44:07
%S 0,1,7,67,127,2107,10003,13741,41523
%N Numbers n such that 9*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 (86*10^n+13)/9 is prime.
%C a(10) > 10^5. - _Robert Price_, Nov 10 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/95557.htm#prime">Prime numbers of the form 955...557</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101011(n-1) + 1, for n>1.
%t Do[ If[ PrimeQ[(86*10^n + 13)/9], Print[n]], {n, 0, 10000}]
%o (PARI) is(n)=isprime(9*10^n + 5*((10^n-1)/9) + 2) \\ _Anders Hellström_, Nov 10 2015
%Y Cf. A002275, A101011.
%K more,nonn
%O 1,3
%A _Robert G. Wilson v_, Jan 19 2005
%E a(7)-a(8) from Kamada data by _Robert Price_, Dec 14 2010
%E a(9) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 17 2013
%E Inserted a(1)=0 by _Robert Price_, Nov 10 2015
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