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 A332149 #8 Feb 11 2020 08:14:24
%S A332149 9,494,44944,4449444,444494444,44444944444,4444449444444,
%T A332149 444444494444444,44444444944444444,4444444449444444444,
%U A332149 444444444494444444444,44444444444944444444444,4444444444449444444444444,444444444444494444444444444,44444444444444944444444444444,4444444444444449444444444444444
%N A332149 a(n) = 4*(10^(2*n+1)-1)/9 + 5*10^n.
%H A332149 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332149 a(n) = 4*A138148(n) + 9*10^n = A002278(2n+1) + 5*10^n.
%F A332149 G.f.: (9 - 505*x + 100*x^2)/((1 - x)(1 - 10*x)(1 - 100*x)).
%F A332149 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332149 A332149 := n -> 4*(10^(2*n+1)-1)/9+5*10^n;
%t A332149 Array[4 (10^(2 # + 1)-1)/9 + 5*10^# &, 15, 0]
%o A332149 (PARI) apply( {A332149(n)=10^(n*2+1)\9*4+5*10^n}, [0..15])
%o A332149 (Python) def A332149(n): return 10**(n*2+1)//9*4+5*10**n
%Y A332149 Cf. A002275 (repunits R_n = (10^n-1)/9), A002278 (4*R_n), A011557 (10^n).
%Y A332149 Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
%Y A332149 Cf. A332119 .. A332189 (variants with different repeated digit 1, ..., 8).
%Y A332149 Cf. A332140 .. A332148 (variants with different middle digit 0, ..., 8).
%K A332149 nonn,base,easy
%O A332149 0,1
%A A332149 _M. F. Hasler_, Feb 09 2020