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 A332125 #7 Feb 11 2020 08:01:17
%S A332125 5,252,22522,2225222,222252222,22222522222,2222225222222,
%T A332125 222222252222222,22222222522222222,2222222225222222222,
%U A332125 222222222252222222222,22222222222522222222222,2222222222225222222222222,222222222222252222222222222,22222222222222522222222222222,2222222222222225222222222222222
%N A332125 a(n) = 2*(10^(2n+1)-1)/9 + 3*10^n.
%H A332125 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332125 a(n) = 2*A138148(n) + 5*10^n = A002276(2n+1) + 3*10^n.
%F A332125 G.f.: (5 - 303*x + 100*x^2)/((1 - x)(1 - 10*x)(1 - 100*x)).
%F A332125 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332125 A332125 := n -> 2*(10^(2*n+1)-1)/9+3*10^n;
%t A332125 Array[2 (10^(2 # + 1)-1)/9 + 3*10^# &, 15, 0]
%o A332125 (PARI) apply( {A332125(n)=10^(n*2+1)\9*2+3*10^n}, [0..15])
%o A332125 (Python) def A332125(n): return 10**(n*2+1)//9*2+3*10**n
%Y A332125 Cf. A002275 (repunits R_n = (10^n-1)/9), A002276 (2*R_n), A011557 (10^n).
%Y A332125 Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
%Y A332125 Cf. A332115 .. A332195 (variants with different repeated digit 1, ..., 9).
%Y A332125 Cf. A332120 .. A332129 (variants with different middle digit 0, ..., 9).
%K A332125 nonn,base,easy
%O A332125 0,1
%A A332125 _M. F. Hasler_, Feb 09 2020