|
%I #40 Jul 08 2021 01:18:58
%S 2,4,5,6,11,26,92,98,110,215,232,354,522,648,1862,2322,2934,3797,6017,
%T 11236,24748,30011,37864,42904,57414,195478
%N Numbers k such that 3*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 (103*10^k - 1)/3 is prime.
%C Terms from Kamada.
%C a(27) > 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/3/34333.htm#prime">Prime numbers of the form 3433...33</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=4, 3*R_6 + 10^4 = 333333 + 10000 = 343333 which is prime.
%t Select[Range[0, 250000], PrimeQ[(103*10^#-1)/3 ] &]
%Y Cf. A002275.
%K more,hard,nonn
%O 1,1
%A _Robert Price_, Apr 14 2015
|
