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 A332121 #8 Feb 11 2020 08:00:16
%S A332121 1,212,22122,2221222,222212222,22222122222,2222221222222,
%T A332121 222222212222222,22222222122222222,2222222221222222222,
%U A332121 222222222212222222222,22222222222122222222222,2222222222221222222222222,222222222222212222222222222,22222222222222122222222222222,2222222222222221222222222222222
%N A332121 a(n) = 2*(10^(2n+1)-1)/9 - 10^n.
%H A332121 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A332121 a(n) = 2*A138148(n) + 1*10^n = A002276(2n+1) - 10^n.
%F A332121 G.f.: (1 + 101*x - 300*x^2)/((1 - x)(1 - 10*x)(1 - 100*x)).
%F A332121 a(n) = 111*a(n-1) - 1110*a(n-2) + 1000*a(n-3) for n > 2.
%p A332121 A332121 := n -> 2*(10^(2*n+1)-1)/9-10^n;
%t A332121 Array[2 (10^(2 # + 1)-1)/9 - 10^# &, 15, 0]
%o A332121 (PARI) apply( {A332121(n)=10^(n*2+1)\9*2-10^n}, [0..15])
%o A332121 (Python) def A332121(n): return 10**(n*2+1)//9*2-10**n
%Y A332121 Cf. A002275 (repunits R_n = (10^n-1)/9), A002276 (2*R_n), A011557 (10^n).
%Y A332121 Cf. A138148 (cyclops numbers with binary digits), A002113 (palindromes).
%Y A332121 Cf. A332120 .. A332129 (variants with different middle digit 0, ..., 9).
%Y A332121 Cf. A332131 .. A332191 (variants with different repeated digit 3, ..., 9).
%K A332121 nonn,base,easy
%O A332121 0,2
%A A332121 _M. F. Hasler_, Feb 09 2020