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 A332127 #7 Feb 11 2020 08:02:39
%S A332127 7,272,22722,2227222,222272222,22222722222,2222227222222,
%T A332127 222222272222222,22222222722222222,2222222227222222222,
%U A332127 222222222272222222222,22222222222722222222222,2222222222227222222222222,222222222222272222222222222,22222222222222722222222222222,2222222222222227222222222222222
%N A332127 a(n) = 2*(10^(2n+1)-1)/9 + 5*10^n.
%H A332127 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332127 a(n) = 2*A138148(n) + 7*10^n = A002276(2n+1) + 5*10^n.
%F A332127 G.f.: (7 - 505*x + 300*x^2)/((1 - x)(1 - 10*x)(1 - 100*x)).
%F A332127 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332127 A332127 := n -> 2*(10^(2*n+1)-1)/9+5*10^n;
%t A332127 Array[2 (10^(2 # + 1)-1)/9 + 5*10^# &, 15, 0]
%o A332127 (PARI) apply( {A332127(n)=10^(n*2+1)\9*2+5*10^n}, [0..15])
%o A332127 (Python) def A332127(n): return 10**(n*2+1)//9*2+5*10**n
%Y A332127 Cf. A002275 (repunits R_n = (10^n-1)/9), A002276 (2*R_n), A011557 (10^n).
%Y A332127 Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
%Y A332127 Cf. A332117 .. A332197 (variants with different repeated digit 1, ..., 9).
%Y A332127 Cf. A332120 .. A332129 (variants with different middle digit 0, ..., 9).
%K A332127 nonn,base,easy
%O A332127 0,1
%A A332127 _M. F. Hasler_, Feb 09 2020