Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #18 Jul 08 2021 06:24:04
%S 19,11119,111119,11111119,11111111111111119,
%T 11111111111111111111111111111111111111111111111119
%N Primes of the form 10*R_k + 9, where R_k is the repunit (A002275) of length k.
%C The next term has 684 digits. - _Harvey P. Dale_, Dec 11 2013
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11119.htm#prime">Prime numbers of the form 11...119</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Select[Table[FromDigits[PadLeft[{9},n,1]],{n,1000}],PrimeQ] (* _Harvey P. Dale_, Dec 11 2013 *)
%Y Cf. A056659 (corresponding k and count of digits 1 in a(n)).
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Mar 28 2004