This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A056725 #41 Jul 19 2021 04:32:04
%S A056725 1,3,7,19,29,37,93,935,8415,9631,11143,41475,41917,48051,107663,
%T A056725 212903,223871,260253,364521,383643,1009567,1762063
%N A056725 Numbers k such that 9*10^k - 1 is prime.
%C A056725 Also numbers k such that 8*10^k + 9*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056725 1009567 is also a member of this sequence, but its position is presently undetermined: 9 * 10^1009567 - 1 is prime. - _Predrag Kurtovic_, Sep 19 2016
%H A056725 Makoto Kamada, <a href="https://stdkmd.net/nrr/8/89999.htm#prime">Prime numbers of the form 899...999</a>.
%H A056725 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056725 Do[ If[ PrimeQ[ 8*10^n + (10^n-1)], Print[n]], {n, 1, 6750, 2}]
%o A056725 (PARI) is(n)=isprime(9*10^n-1) \\ _Charles R Greathouse IV_, Feb 17 2017
%Y A056725 Cf. A003307, A002235, A046865, A079906, A046866, A001771, A005541, A046867, A079907.
%K A056725 hard,nonn,more
%O A056725 1,2
%A A056725 _Robert G. Wilson v_, Aug 11 2000
%E A056725 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A056725 a(16)-a(20) from Kamada data by _Robert Price_, Oct 19 2014
%E A056725 a(21) from Kamada data by _Robert Price_, Mar 10 2019
%E A056725 a(22) from Kamada data by _Mohammed Yaseen_, Jul 18 2021