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 A286916 #78 Jul 24 2021 02:25:38
%S A286916 7,13,23,409,1147,2597,4561
%N A286916 Positive numbers k such that (10^(k+2)*331 - 133)/9 is prime.
%C A286916 Or '36'||...'7'...||'63' in decimal form is prime ('7' concatenated k times to which the prefix '36' and the suffix '63' are concatenated once).
%C A286916 a(1)..a(4) and a(7) themselves are primes. a(8) > 50000 (if it exists).
%e A286916 7 is a term as 36777777763 is prime (as a string, it consists of '7' concatenated 7 times to which the prefix '36' and the suffix '63' are concatenated once).
%t A286916 ParallelMap[ If[ PrimeQ[ (10^(#+2)*331-133)/9], #, Nothing]&, Range[5000]]
%o A286916 (PARI) is(n)=ispseudoprime((10^(n+2)*331-133)/9) \\ _Charles R Greathouse IV_, Jun 13 2017
%K A286916 nonn,hard,more,base
%O A286916 1,1
%A A286916 _Mikk Heidemaa_, May 17 2017