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%I #22 Sep 08 2022 08:46:12
%S 3,4,5,9,19,25,34,70,82,718,2035,2494,3369,3953,4219,8524,30943,49375,
%T 52082,139670,303255
%N Numbers k such that 9*R_(k+2) - 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that 99*10^k - 1 is prime.
%C Terms from Kamada.
%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/9/98999.htm#prime">Prime numbers of the form 9899...99</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=3, 9*R_5 - 10^3 = 99999 - 1000 = 98999 which is prime.
%t Select[Range[0, 280000], PrimeQ[99*10^#-1 ] &]
%o (Magma) [n: n in [0..300] | IsPrime(99*10^n-1)]; // _Vincenzo Librandi_, Apr 15 2015
%Y Cf. A002275.
%K more,hard,nonn
%O 1,1
%A _Robert Price_, Apr 14 2015
%E a(21) from _Predrag Kurtovic_, May 24 2015
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