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%I #18 Jul 08 2021 03:15:04
%S 1,2,3,6,46,394,978,2586,2811,2968,3642,4827,4918,5592,5706,10683,
%T 12891,14118,74350,88680
%N Numbers k such that 3*R_k + 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (10^k + 11)/3 is prime.
%C a(21) > 10^5. - _Robert Price_, Nov 02 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/33337.htm#prime">Prime numbers of the form 33...337</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A056680(n) + 1.
%t Do[ If[ PrimeQ[ 3(10^n - 1)/9 + 4], Print[n]], {n, 10000}]
%Y Cf. A002275, A056680, A093168.
%K hard,nonn
%O 1,2
%A _Robert G. Wilson v_, Oct 14 2004
%E a(16)-a(20) from _Robert Price_, Nov 02 2014
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