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%I #23 Jan 17 2019 13:44:07
%S 1,2,4,7,10,12,20,24,30,42,62,188,304,340,715,804,1040,1887,2079,2120,
%T 2260,2423,3319,5598,6072,22048,25713,28644,43353
%N Numbers n such that 4*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 (13*10^n-7)/3 is prime.
%C a(30) > 10^5. - _Robert Price_, Mar 30 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/43331.htm#prime">Prime numbers of the form 433...331</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101723(n) + 1.
%t Do[ If[ PrimeQ[(13*10^n - 7)/3], Print[n]], {n, 1, 10000}]
%Y Cf. A002275, A101723.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 17 2005
%E Addition of a(26)-a(28) from Kamada data by _Robert Price_, Dec 08 2010
%E a(29) from Erik Branger, May 01 2013, submitted by _Ray Chandler_, Aug 16 2013
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