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 A213790 #24 Aug 12 2024 18:36:09
%S A213790 1,10,1,2,2,3,1,6,2,20,3,4,30,9,4,5,14,4,1,658,6,10,32,9,2,9,109,8,1,
%T A213790 7,12,6,4,2,5,137,1,15,112,30,237,83,12,21,5,4,20,15,42,3,16,41,26,60,
%U A213790 157,8,16,76,69,10,4,4,120,39,8,7,115,22,14,2,102
%N A213790 Least number k such that 10^(n+k) - 10^n - 1 is prime.
%C A213790 Near repdigit primes: concatenation of k-1 9's, one 8, and n 9's.
%H A213790 Pierre CAMI, <a href="/A213790/b213790.txt">Table of n, a(n) for n = 1..1100</a>
%e A213790 (10^1-1)*10^1-1 = 89 prime so a(1) = 1.
%e A213790 (10^10-1)*10^2-1 = 999999999899 prime so a(2) = 10.
%t A213790 lnk[n_]:=Module[{k=1,c=10^n+1},While[!PrimeQ[10^(n+k)-c],k++];k]; Array[lnk,80] (* _Harvey P. Dale_, Aug 12 2024 *)
%o A213790 (PARI) a(n) = {my(k=1); while (!ispseudoprime(10^(n+k) - 10^n - 1), k++); k;} \\ _Michel Marcus_, Sep 21 2019
%K A213790 nonn
%O A213790 1,2
%A A213790 _Pierre CAMI_, Jun 20 2012