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 A216346 #8 Jun 02 2025 08:06:55
%S A216346 199,2999,799999999,1099999999999,14999999999999999,
%T A216346 38999999999999999999999999999999999999999,
%U A216346 59999999999999999999999999999999999999999999999999999999999999,71999999999999999999999999999999999999999999999999999999999999999999999999
%N A216346 Primes of the form n*10^n - 1.
%C A216346 The corresponding n are in A059671.
%C A216346 a(9) = 769999999….999999 contains 79 digits.
%C A216346 a(10) = 1169999999….999999 contains 120 digits.
%t A216346 lst={}; Do[ p=n*10^n- 1; If[PrimeQ[p], AppendTo[lst, p]], {n, 0, 60}]; lst
%t A216346 Select[Table[n*10^n-1,{n,100}],PrimeQ] (* _Harvey P. Dale_, Sep 17 2023 *)
%Y A216346 Cf. A059671.
%K A216346 nonn
%O A216346 1,1
%A A216346 _Michel Lagneau_, Sep 04 2012