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 A332146 #6 Feb 11 2020 08:09:22
%S A332146 6,464,44644,4446444,444464444,44444644444,4444446444444,
%T A332146 444444464444444,44444444644444444,4444444446444444444,
%U A332146 444444444464444444444,44444444444644444444444,4444444444446444444444444,444444444444464444444444444,44444444444444644444444444444,4444444444444446444444444444444
%N A332146 a(n) = 4*(10^(2*n+1)-1)/9 + 2*10^n.
%H A332146 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332146 a(n) = 4*A138148(n) + 6*10^n = A002278(2n+1) + 2*10^n = 2*A332123(n).
%F A332146 G.f.: (6 - 202*x - 200*x^2)/((1 - x)(1 - 10*x)(1 - 100*x)).
%F A332146 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332146 A332146 := n -> 4*(10^(2*n+1)-1)/9+2*10^n;
%t A332146 Array[4 (10^(2 # + 1)-1)/9 + 2*10^# &, 15, 0]
%o A332146 (PARI) apply( {A332146(n)=10^(n*2+1)\9*4+2*10^n}, [0..15])
%o A332146 (Python) def A332146(n): return 10**(n*2+1)//9*4+2*10**n
%Y A332146 Cf. A002275 (repunits R_n = (10^n-1)/9), A002278 (4*R_n), A011557 (10^n).
%Y A332146 Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
%Y A332146 Cf. A332116 .. A332196 (variants with different repeated digit 2, ..., 9).
%Y A332146 Cf. A332140 .. A332149 (variants with different middle digit 0, ..., 9).
%K A332146 nonn,base,easy
%O A332146 0,1
%A A332146 _M. F. Hasler_, Feb 09 2020