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%I #23 Sep 08 2022 08:45:15
%S 0,3,5,6,48,108,245,1044,20208,52740,89189
%N Numbers k such that 4*R_k + 5 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (4*10^k + 41)/9 is prime.
%C a(12) > 10^5. - _Robert Price_, Nov 09 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44449.htm#prime">Prime numbers of the form 44...449</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A056683(n-1) + 1.
%t Do[ If[ PrimeQ[ 4(10^n - 1)/9 + 5], Print[n]], {n, 5000}]
%o (Magma) [n: n in [0..300] | IsPrime((4*10^n+41) div 9)]; // _Vincenzo Librandi_ Nov 10 2014
%Y Cf. A002275, A056683.
%K nonn,more
%O 1,2
%A _Robert G. Wilson v_, Oct 14 2004
%E a(9)-a(11) from _Robert Price_, Nov 09 2014
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