|
%I #24 Jul 08 2021 01:20:14
%S 1,2,4,16,23,25,27,29,34,68,89,127,200,418,868,1173,2553,4131,5675,
%T 8318,8445,10820,15793,16671,32275,43995,48026,50035,88459,126839
%N Numbers k such that 3*R_(k+2) + 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that (106*10^k - 1)/3 is prime.
%C Terms from Kamada.
%C a(31) > 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/3/35333.htm#prime">Prime numbers of the form 3533...33</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=4, 3*R_6 + 2*10^4 = 333333 + 20000 = 353333 which is prime.
%t Select[Range[0, 250000], PrimeQ[(106*10^#-1)/3 ] &]
%Y Cf. A002275.
%K more,hard,nonn
%O 1,2
%A _Robert Price_, Apr 14 2015
|
