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 A332190 #17 May 28 2021 16:56:25
%S A332190 0,909,99099,9990999,999909999,99999099999,9999990999999,
%T A332190 999999909999999,99999999099999999,9999999990999999999,
%U A332190 999999999909999999999,99999999999099999999999,9999999999990999999999999,999999999999909999999999999,99999999999999099999999999999,9999999999999990999999999999999
%N A332190 a(n) = 10^(2n+1) - 1 - 9*10^n.
%H A332190 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332190 a(n) = 9*A138148(n) = A002283(2n+1) - A011557(n).
%F A332190 G.f.: 9*x*(101 - 200*x)/((1 - x)(1 - 10*x)(1 - 100*x)).
%F A332190 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332190 A332190 := n -> 10^(2*n+1)-1-9*10^n;
%t A332190 Array[10^(2 # + 1)-1-9*10^# &, 15, 0]
%t A332190 LinearRecurrence[{111,-1110,1000},{0,909,99099},20] (* _Harvey P. Dale_, May 28 2021 *)
%o A332190 (PARI) apply( {A332190(n)=10^(n*2+1)-1-9*10^n}, [0..15])
%o A332190 (Python) def A332190(n): return 10**(n*2+1)-1-9*10^n
%Y A332190 Cf. A002275 (repunits R_n = (10^n-1)/9), A002283 (9*R_n), A011557 (10^n).
%Y A332190 Cf. A138148 (cyclops numbers with binary digits only), A002113 (palindromes).
%Y A332190 Cf. A332120 .. A332180 (variants with different repeated digit 2, ..., 8).
%Y A332190 Cf. A332191 .. A332197, A181965 (variants with different middle digit 1, ..., 8).
%K A332190 nonn,base,easy
%O A332190 0,2
%A A332190 _M. F. Hasler_, Feb 08 2020