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 A287017 #29 Jul 24 2021 02:25:09
%S A287017 5,13,41,107,3049,52727
%N A287017 Positive numbers k such that (10^(k+2)*109 + 89)/9 is prime.
%C A287017 Or '12'||...'1'...||'21' in decimal form is prime ('1' concatenated k times to which the prefix '12' and the suffix '21' are concatenated once).
%C A287017 a(1)...a(6) themselves are primes. a(7) > 70000 (if it exists).
%e A287017 5 is a term as 121111121 is prime (as a string, it consists of '1' concatenated 5 times to which the prefix '12' and the suffix '21' are concatenated once).
%t A287017 ParallelMap[ If[ PrimeQ[ (10^(#+2)*109+89)/9], #, Nothing]&, Range[3500]]
%o A287017 (PARI) is(n)=ispseudoprime((10^(n+2)*109+89)/9) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A287017 Cf. A286916, A210873.
%K A287017 nonn,hard,more,base
%O A287017 1,1
%A A287017 _Mikk Heidemaa_, May 19 2017