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 A287061 #39 Jul 22 2021 23:28:11
%S A287061 11,19,23,31,73,257,613,5327,62359
%N A287061 Numbers k such that (10^(k+3)*6319 + 863)/9 is prime (k > 0).
%C A287061 The generated prime numbers are of the form '702'||...'1'...||'207' ('1' concatenated k times to which the prefix '702' and the suffix '207' are concatenated once).
%C A287061 a(1)..a(7) themselves are primes. a(10) > 80000, if it exists.
%e A287061 11 is a term as (10^(11+3)*6319 + 863)/9 = 70211111111111207 is prime.
%e A287061 23 is a term because (10^(23+3)*6319 + 863)/9 = 70211111111111111111111111207 is prime.
%t A287061 ParallelMap[ If[ PrimeQ[ (10^(#+3)*6319+863)/9], #, Nothing]&, Range[1000]]
%o A287061 (PARI) is(n) = ispseudoprime((10^(n+3)*6319+863)/9); \\ _Altug Alkan_, Jul 23 2017
%Y A287061 Cf. A032674, A260405, A286916.
%K A287061 nonn,base,hard,more
%O A287061 1,1
%A A287061 _Mikk Heidemaa_, May 19 2017