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 A205084 #24 Feb 18 2025 16:34:41
%S A205084 11,141,1441,14441,144441,1444441,14444441,144444441,1444444441,
%T A205084 14444444441,144444444441,1444444444441,14444444444441,
%U A205084 144444444444441,1444444444444441,14444444444444441,144444444444444441,1444444444444444441,14444444444444444441
%N A205084 a(n) = n 4's sandwiched between two 1's.
%H A205084 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (11,-10).
%F A205084 a(0)=11, a(n) = 10*a(n-1) + 31.
%F A205084 a(n) = (130*10^n - 31)/9 (see PARI's code by Charles R Greathouse IV).
%F A205084 From _Elmo R. Oliveira_, Feb 18 2025: (Start)
%F A205084 G.f.: (11 + 20*x)/((1 - x)*(1 - 10*x)).
%F A205084 E.g.f.: exp(x)*(130*exp(9*x) - 31)/9.
%F A205084 a(n) = 11*a(n-1) - 10*a(n-2). (End)
%t A205084 a[0]=11;a[n_]:=a[n-1]*10+31;Table[a[n],{n,0,44}]
%o A205084 (PARI) a(n)=(130*10^n-31)/9 \\ _Charles R Greathouse IV_, Jan 23 2012
%Y A205084 Cf. A002278.
%K A205084 nonn,easy,base,less
%O A205084 0,1
%A A205084 _José María Grau Ribas_, Jan 22 2012