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%I #16 Jan 17 2019 13:44:07
%S 2,9,12,15,26,45,62,80,128,326,494,614,710,1641,1646,2103,8514,8688
%N Numbers n such that 5*10^n + 2*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (47*10^n-11)/9 is prime.
%C No other terms below 50000.
%C a(19) > 10^5. - _Robert Price_, Jul 04 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/52221.htm#prime">Prime numbers of the form 522...221</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101573(n) + 1.
%t Do[ If[ PrimeQ[(47*10^n - 11)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101573.
%K more,nonn
%O 1,1
%A _Robert G. Wilson v_, Jan 18 2005
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