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 A111391 #15 Apr 14 2024 03:44:59
%S A111391 1,9,11,17,22,29,36,37,52,166,448,2011,3489,4871,6982,10024,16974,
%T A111391 33287,47364,58873,126160,234424,382881,524706
%N A111391 Numbers k such that 11*10^k - 1 is prime.
%C A111391 Numbers k such that 10^(k+1) + (10^k-1) is a prime.
%H A111391 Gary Barnes, <a href="http://www.noprimeleftbehind.net/gary/primes-kx10n-1.htm">Primes of the form k*10^n-1</a>
%H A111391 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 109w.</a>
%t A111391 Do[ If[ PrimeQ[10^(n + 1) + (10^n - 1)], Print[n]], {n, 9500}]
%o A111391 (PARI) is(n)=ispseudoprime(11*10^n-1) \\ _Charles R Greathouse IV_, Jun 12 2017
%Y A111391 Cf. A096209.
%K A111391 more,nonn
%O A111391 1,2
%A A111391 _Robert G. Wilson v_, Nov 15 2005
%E A111391 a(16)-a(19) from _Ray Chandler_, Sep 16 2013
%E A111391 a(20)-a(21) from P. Kurtovic submitted by _Ray Chandler_, Sep 17 2013
%E A111391 a(22)-a(24) from Kamada data by _Tyler Busby_, Apr 14 2024