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%I #30 Sep 08 2022 08:45:15
%S 0,1,3,9,15,28,64,1168,1695,2362
%N Numbers k such that 2*R_k + 5 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (2*10^k + 43)/9 is prime.
%C a(11) > 50000. - _Robert Price_, Oct 27 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/22227.htm#prime">Prime numbers of the form 22...227</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A056677(n-1) + 1.
%t Do[ If[ PrimeQ[ 2(10^n - 1)/9 + 5], Print[n]], {n, 0, 5000}]
%o (Magma) [n: n in [0..1000] | IsPrime( 2*(10^n - 1) div 9 + 5)]; // _Vincenzo Librandi_, Oct 28 2014
%Y Cf. A002275, A056677, A093167.
%K nonn
%O 1,3
%A _Robert G. Wilson v_, Oct 14 2004
%E Added a(1)=0, adapted Mathematica program, _Vincenzo Librandi_, Oct 28 2014
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