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 A028987 #27 Jan 26 2023 12:12:22
%S A028987 3,4,6,8,44,444,888,2222,8888,444444,888888,444444444,888888888,
%T A028987 444444444444,888888888888,222222222222222222,
%U A028987 444444444444444444444444444444,44444444444444444444444444444444
%N A028987 Repdigit - 1 is prime.
%C A028987 Next term is 88...8 (72 digits).
%C A028987 Corresponding values of primes are in A096843. - _Jaroslav Krizek_, Mar 19 2013
%H A028987 Patrick De Geest, <a href="http://www.worldofnumbers.com/em83.htm">Repdigits : WONplate 83</a>
%t A028987 a[n_]:=NestList[FromDigits[Append[{#},n]]&,n,35]; Union[Join[{3},Flatten[Table[Select[a[n],PrimeQ[#-1]&],{n,2,8,2}]]]] (* _Jayanta Basu_, May 29 2013 *)
%o A028987 (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, ", ")))) \\ _Felix Fröhlich_, Jul 07 2014
%Y A028987 Cf. A010785.
%K A028987 nonn,base
%O A028987 1,1
%A A028987 _Patrick De Geest_
%E A028987 Offset corrected and initial term added by _Arkadiusz Wesolowski_, Aug 14 2011