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%I #17 Jan 17 2019 13:44:07
%S 0,1,2,4,7,22,23,80,4427,31319,80150,91144
%N Numbers n such that 4*10^n + 2*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (38*10^n-11)/9 is prime.
%C a(13) > 10^5. - _Robert Price_, May 08 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/42221.htm#prime">Prime numbers of the form 422...221</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101719(n) + 1.
%t Do[ If[ PrimeQ[(38*10^n - 11)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101719.
%K more,nonn
%O 1,3
%A _Robert G. Wilson v_, Dec 22 2004
%E a(10) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
%E a(1) = 0 from _Robert Price_, May 08 2015
%E a(11)-a(12) from _Robert Price_, May 08 2015
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