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%I #19 Jan 17 2019 13:44:07
%S 1,2,4,7,18,19,23,24,34,41,56,64,84,149,272,755,1272,2398,2686,4800,
%T 5198,6217,8737,12388,12391,19702,42404
%N Numbers n such that 6*10^n + 3*R_n - 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (19*10^n-7)/3 is prime.
%C a(28) > 10^5. - _Robert Price_, Sep 10 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/6/63331.htm#prime">Prime numbers of the form 633...331</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101525(n) + 1.
%t Do[ If[ PrimeQ[(19*10^n - 7)/3], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101525.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 18 2005
%E Addition of a(24)-a(26) from Kamada data by _Robert Price_, Dec 09 2010
%E a(27) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
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