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%I #25 Jul 08 2021 03:09:41
%S 1,2,8,10,25,50,76,104,4406,8935,26000,31652
%N Numbers k such that 10^k + 7*R_k + 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (16*10^k + 11)/9 is prime.
%C a(13) > 10^5. - _Robert Price_, Feb 07 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/17779.htm#prime">Prime numbers of the form 177...779</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102028(n) - 1.
%t Do[ If[ PrimeQ[(16*10^n + 11)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A102028.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Dec 16 2004
%E Addition of a(11) from Kamada data by _Robert Price_, Dec 13 2010
%E a(12) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
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