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%I #22 Jan 17 2019 13:44:07
%S 6,21,22,24,51,81,132,165,621,1450,3424,5979,11526,26980,49233,84927
%N Numbers n such that 4*10^n + 5*R_n + 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (41*10^n+31)/9 is prime.
%C a(17) > 10^5. - _Robert Price_, May 25 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/45559.htm#prime">Prime numbers of the form 455...559</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101728(n) + 1.
%t Do[ If[ PrimeQ[(41*10^n + 31)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101728.
%K more,nonn
%O 1,1
%A _Robert G. Wilson v_, Jan 17 2005
%E Addition of a(13)-a(14) from Kamada data by _Robert Price_, Dec 12 2010
%E a(15) from Erik Branger, May 01 2013, submitted by _Ray Chandler_, Aug 16 2013
%E a(16) from _Robert Price_, May 25 2015
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