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 A108904 #23 Sep 03 2024 00:44:30
%S A108904 99,979,9779,97779,977779,9777779,97777779,977777779,9777777779,
%T A108904 97777777779,977777777779,9777777777779,97777777777779,
%U A108904 977777777777779,9777777777777779,97777777777777779,977777777777777779,9777777777777777779,97777777777777777779,977777777777777777779
%N A108904 a(n) has two outer digits 9 and n inner digits 7.
%C A108904 These numbers are all composite. They are divisible by 3,7 or 11.
%C A108904 All terms are divisible by 11. - _Jason Yuen_, Sep 02 2024
%H A108904 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A108904 a(n) = 9*10^(n+1) + 9 + 70*(10^n-1)/9.
%F A108904 a(n) = 11*(80*10^n + 1)/9. - _Jason Yuen_, Sep 02 2024
%F A108904 From _Alois P. Heinz_, Sep 02 2024: (Start)
%F A108904 a(n) = 11*A059482(n+1).
%F A108904 G.f.: 11*(9-10*x)/((10*x-1)*(x-1)). (End)
%o A108904 (PARI) g(n,r=7,m=9) = /* Repeat rrr.. surrounded by 2 m's */ { for(x=0,n, y=m*10^(x+1)+m+r*10*(10^x-1)/9; print1(y",") ) }
%Y A108904 Cf. A059482.
%K A108904 easy,nonn,base
%O A108904 0,1
%A A108904 _Cino Hilliard_, Jul 16 2005
%E A108904 Minor edits by _N. J. A. Sloane_, Aug 01 2010
%E A108904 a(0) prepended by _Alois P. Heinz_, Sep 02 2024