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%I #16 Jan 17 2019 13:44:07
%S 1,4,12,13,15,36,40,90,837,2424,2427,2443,15925,16212
%N Numbers n such that 7*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 (65*10^n+7)/9 is prime.
%C a(15) > 10^5. - _Robert Price_, Sep 22 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/72223.htm#prime">Prime numbers of the form 722...223</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101134(n) + 1.
%t Do[ If[ PrimeQ[(65*10^n + 7)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101134.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 19 2005
%E a(13)-a(14) from Kamada data by _Robert Price_, Dec 14 2010
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