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 A332140 #11 Jul 06 2021 16:03:13
%S A332140 0,404,44044,4440444,444404444,44444044444,4444440444444,
%T A332140 444444404444444,44444444044444444,4444444440444444444,
%U A332140 444444444404444444444,44444444444044444444444,4444444444440444444444444,444444444444404444444444444,44444444444444044444444444444,4444444444444440444444444444444
%N A332140 a(n) = 4*(10^(2n+1)-1)/9 - 4*10^n.
%H A332140 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332140 a(n) = 4*A138148(n) = A002278(2n+1) - 4*10^n.
%F A332140 G.f.: 4*x*(101 - 200*x)/((1 - x)(1 - 10*x)(1 - 100*x)).
%F A332140 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332140 A332140 := n -> 4*((10^(2*n+1)-1)/9-10^n);
%t A332140 Array[4 ((10^(2 # + 1)-1)/9 - 10^#) &, 15, 0]
%t A332140 LinearRecurrence[{111,-1110,1000},{0,404,44044},20] (* _Harvey P. Dale_, Jul 06 2021 *)
%o A332140 (PARI) apply( {A332140(n)=(10^(n*2+1)\9-10^n)*4}, [0..15])
%o A332140 (Python) def A332140(n): return (10**(n*2+1)//9-10**n)*4
%Y A332140 Cf. A002275 (repunits R_n = (10^n-1)/9), A002278 (4*R_n), A011557 (10^n).
%Y A332140 Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
%Y A332140 Cf. A332120 .. A332190 (variants with different repeated digit 2, ..., 9).
%Y A332140 Cf. A332141 .. A332149 (variants with different middle digit 1, ..., 9).
%K A332140 nonn,base,easy
%O A332140 0,2
%A A332140 _M. F. Hasler_, Feb 09 2020