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 A028988 #21 Jan 27 2023 08:11:24
%S A028988 1,2,4,6,22,66,88,222,666666,22222222,66666666,666666666,22222222222,
%T A028988 66666666666,88888888888888,88888888888888888,66666666666666666666,
%U A028988 66666666666666666666666,88888888888888888888888888888888888
%N A028988 Repdigit + 1 is prime.
%C A028988 Next term is 22...2 (36 digits).
%H A028988 Michel Marcus, <a href="/A028988/b028988.txt">Table of n, a(n) for n = 1..39</a>
%H A028988 Patrick De Geest, <a href="http://www.worldofnumbers.com/em83.htm">Repdigits: WONplate 83</a>
%t A028988 a[n_]:=NestList[FromDigits[Append[{#},n]]&,n,34]; Union[Join[{1},Flatten[Table[Select[a[n],PrimeQ[#+1]&],{n,2,8,2}]]]] (* _Jayanta Basu_, May 30 2013 *)
%t A028988 Select[Flatten[Table[FromDigits[PadRight[{},n,k]],{n,40},{k,9}]],PrimeQ[ #+1]&] (* _Harvey P. Dale_, Apr 22 2018 *)
%o A028988 (PARI) u=30; for(n=1, u, r=(10^n-1)/9; for(a=1, 9, m=r*a; if(ispseudoprime(m+1), print1(m, ", ")))) \\ _Michel Marcus_, Jan 26 2023; after A028987
%Y A028988 Cf. A010785, A028987.
%K A028988 nonn,base
%O A028988 1,2
%A A028988 _Patrick De Geest_
%E A028988 Offset corrected by _Arkadiusz Wesolowski_, Aug 14 2011