%I #31 Jul 08 2021 05:05:42

%S 2,5,20,41,47,92,161,401,455,8570,18592,73714,78635,100780

%N Numbers k such that 9*10^k + R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (82*10^k - 1)/9 is prime.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/91111.htm#prime">Prime numbers of the form 911...11</a>.

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

%t Do[ If[ PrimeQ[ 9*10^n + (10^n-1)/9], Print[n]], {n, 0, 5000}]

%Y Cf. A002275, A093634.

%K hard,nonn

%O 1,1

%A _Robert G. Wilson v_, Aug 11 2000

%E 8570 from _Hugo Pfoertner_, Oct 22 2004

%E a(11)-a(14) from _Robert Price_, Dec 05 2014