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 A205085 #30 Feb 18 2025 16:17:02
%S A205085 11,151,1551,15551,155551,1555551,15555551,155555551,1555555551,
%T A205085 15555555551,155555555551,1555555555551,15555555555551,
%U A205085 155555555555551,1555555555555551,15555555555555551,155555555555555551,1555555555555555551,15555555555555555551,155555555555555555551
%N A205085 a(n) = n 5's sandwiched between two 1's.
%H A205085 Vincenzo Librandi, <a href="/A205085/b205085.txt">Table of n, a(n) for n = 0..100</a>
%H A205085 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A205085 a(0)=11, a(n) = 10*a(n-1) + 41.
%F A205085 a(n) = (140*10^n - 41)/9 (see PARI code by _Charles R Greathouse IV_).
%F A205085 a(n) = 11*a(n-1) - 10*a(n-2). - _Vincenzo Librandi_, Jan 23 2012
%F A205085 From _Elmo R. Oliveira_, Feb 18 2025: (Start)
%F A205085 G.f.: (11 + 30*x)/((1 - x)*(1 - 10*x)).
%F A205085 E.g.f.: exp(x)*(140*exp(9*x) - 41)/9. (End)
%t A205085 a[0]=11;a[n_]:=a[n-1]*10+41;Table[a[n],{n,0,44}]
%t A205085 LinearRecurrence[{11, -10}, {11, 151}, 50] (* _Vincenzo Librandi_, Jan 23 2012 *)
%t A205085 Table[10FromDigits[PadRight[{1},n,5]]+1,{n,20}] (* _Harvey P. Dale_, May 02 2019 *)
%o A205085 (PARI) a(n)=(140*10^n-41)/9 \\ _Charles R Greathouse IV_, Jan 23 2012
%o A205085 (Magma) I:=[11, 151]; [n le 2 select I[n] else 11*Self(n-1)-10*Self(n-2): n in [1..25]]; // _Vincenzo Librandi_, Jan 23 2012
%Y A205085 Cf. A002279.
%K A205085 nonn,base,easy,less
%O A205085 0,1
%A A205085 _José María Grau Ribas_, Jan 22 2012