A256932 - OEIS

%I #17 Sep 08 2022 08:46:12

%S 0,8,9,23,54,72,92,402,405,651,860,897,2691,3887,4665,4971,5271,8931,

%T 9953,10566,10907,11955,22467,66755,153810,169337,224061

%N Numbers k such that R_(k+2) + 6*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also, numbers k such that (154*10^k - 1)/9  is prime.

%C Terms from Kamada.

%C a(28) > 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/1/17111.htm#prime">Prime numbers of the form 1711...11</a>.

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

%e For k=8, R_10 + 6*10^8 = 1111111111 + 600000000 = 1711111111 which is prime.

%t Select[Range[0, 250000], PrimeQ[(154*10^#-1)/9 ] &]

%o (Magma) [n: n in [0..300] | IsPrime((154*10^n-1) div 9)]; // _Vincenzo Librandi_, Apr 14 2015

%o (PARI) for(n=0,300,if(isprime((154*10^n-1)/9),print1(n,", "))) \\ _Derek Orr_, Apr 14 2015

%Y Cf. A002275.

%K more,hard,nonn

%O 1,2

%A _Robert Price_, Apr 13 2015