A256800 - OEIS

%I #19 Sep 08 2022 08:46:12

%S 1,2,3,6,7,13,22,28,32,126,172,186,267,650,693,1083,3783,12294,18134,

%T 53859,66650,72097,98890,125706,200001

%N Numbers k such that 3*R_k + 50 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also, numbers k such that (10^k + 149)/3 is prime.

%C Terms from Kamada data. Note that Kamada does not recognize k=1 as 53 is a degenerate case of form AAA..ABA.

%C a(26) > 2.5*10^5.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/aaaba.htm">Near-repdigit numbers of the form AA...AABA</a>.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/33383.htm#prime">Prime numbers of the form 33...3383</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.

%e For k=3, 3*R_3 + 50 = 333 + 50 = 383 which is prime.

%t Select[Range[0, 250000], PrimeQ[(10^# + 149)/3] &]

%o (Magma) [n: n in [0..300] | IsPrime((10^n+149) div 3)]; // _Vincenzo Librandi_, Apr 11 2015

%Y Cf. A002275.

%K more,hard,nonn

%O 1,2

%A _Robert Price_, Apr 10 2015