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 A287766 #38 Jul 22 2021 23:24:52
%S A287766 1,9,13,33,81,1113,1609,5697,13577,49949
%N A287766 Numbers k such that (10^k*113 + 19)/3 is prime.
%C A287766 For k > 2, numbers such that '37'||...'6'...||'73' ('6' concatenated k-2 times and prefixed by '37', and suffixed by '73') in decimal form is prime.
%e A287766 13 is in the sequence because (10^13*113 + 19)/3 = 376666666666673 is prime.
%t A287766 ParallelTable[ If[ PrimeQ[ (10^n*113+19)/3], n, Nothing], {n, 2000}]
%o A287766 (PARI) isok(n) = isprime((10^n*113 + 19)/3); \\ _Michel Marcus_, Jul 21 2017
%Y A287766 Cf. A051919, A101724.
%K A287766 nonn,hard,more
%O A287766 1,2
%A A287766 _Mikk Heidemaa_, May 31 2017