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%I #16 Sep 08 2022 08:46:12
%S 1,2,22,280,944,1376,1837,2329,2512,5186,24454,25354,49774,103474,
%T 159736,177754
%N Numbers k such that R_(k+2) + 4*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that (136*10^k - 1)/9 is prime.
%C Terms from Kamada.
%C a(17) > 250000.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/abaaa.htm">Near-repdigit numbers of the form ABAA...AA</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/15111.htm#prime">Prime numbers of the form 1511...11</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=2, R_4 + 4*10^2 = 1111 + 400 = 1511 which is prime.
%t Select[Range[0, 250000], PrimeQ[(136*10^#-1)/9 ] &]
%o (Magma) [n: n in [0..300] | IsPrime((136*10^n-1) div 9)]; // _Vincenzo Librandi_, Apr 14 2015
%o (PARI) for(n=0,300,if(isprime((136*10^n-1)/9),print1(n,", "))) \\ _Derek Orr_, Apr 14 2015
%Y Cf. A002275.
%K more,hard,nonn
%O 1,2
%A _Robert Price_, Apr 13 2015
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