%I #21 Sep 08 2022 08:46:12
%S 0,66,84,3366,14934
%N Numbers k such that 7*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 (664*10^k - 7)/9 is prime.
%C Terms from Kamada.
%C a(6) > 30000.
%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/7/73777.htm#prime">Prime numbers of the form 7377...77</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%t Select[Range[0, 30000], PrimeQ[(664*10^#-7)/9 ] &]
%o (PARI) for(n=0,100,if(isprime((664*10^n-7)/9),print1(n,", "))) \\ _Derek Orr_, Apr 14 2015
%o (Magma) [n: n in [0..400] | IsPrime((664*10^n-7) div 9)]; // _Vincenzo Librandi_, Apr 15 2015
%Y Cf. A002275.
%K more,hard,nonn
%O 1,2
%A _Robert Price_, Apr 14 2015