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 A070220 #8 Jun 08 2021 18:01:29
%S A070220 11,211,511111,71111111,101111111111,161111111111111111,
%T A070220 2011111111111111111111,
%U A070220 6511111111111111111111111111111111111111111111111111111111111111111
%N A070220 Primes of the form n (written in base 10) followed by n 1's.
%t A070220 Do[p = FromDigits[ Join[ IntegerDigits[n], IntegerDigits[(10^n - 1)/9]]]; If[ PrimeQ[p], Print[p]], {n, 1, 100}]
%t A070220 Select[Table[FromDigits[Join[IntegerDigits[n],PadRight[{},n,1]]],{n,100}],PrimeQ] (* _Harvey P. Dale_, Jun 08 2021 *)
%Y A070220 Cf. A068817 (for values of n).
%K A070220 base,nonn
%O A070220 1,1
%A A070220 _Amarnath Murthy_, May 02 2002
%E A070220 More terms from _Vladeta Jovovic_ and _Robert G. Wilson v_, May 03 2002