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%I #16 Jan 17 2019 13:44:07
%S 2,6,8,18,53,68,242,276,320,564,620,1782,2340,2454,5765,15530,15605,
%T 22938
%N Numbers n such that 5*10^n + 4*R_n + 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (49*10^n+23)/9 is prime.
%C a(19) > 10^5. - _Robert Price_, Jul 29 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/54447.htm#prime">Prime numbers of the form 544...447</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101580(n) + 1.
%t Do[ If[ PrimeQ[(49*10^n + 23)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101580.
%K more,nonn
%O 1,1
%A _Robert G. Wilson v_, Jan 18 2005
%E a(16)-a(18) from Kamada data by _Robert Price_, Dec 14 2010
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