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 A325906 #26 May 07 2021 05:09:36
%S A325906 1,12,1121,11112212,1111111122221121,11111111111111112222222211112212,
%T A325906 1111111111111111111111111111111122222222222222221111111122221121
%N A325906 a(n) = ( (-1)^n * Sum_{k=0..n-2} (-1)^k*10^(2^k) + 10^(2^(n-1)) - ((-1)^n+3)/2 )/9.
%H A325906 Seiichi Manyama, <a href="/A325906/b325906.txt">Table of n, a(n) for n = 1..10</a>
%e A325906 n | a(n) | A325910(n)
%e A325906 --+------------------+-----------------
%e A325906 1 | 1 | 1
%e A325906 2 | 12 | 10
%e A325906 3 | 1121 | 1101
%e A325906 4 | 11112212 | 11110010
%e A325906 5 | 1111111122221121 | 1111111100001101
%t A325906 a[n_] := ((-1)^n * Sum[(-1)^k * 10^(2^k), {k, 0, n - 2}] + 10^(2^(n - 1)) - ((-1)^n + 3)/2)/9; Array[a, 7] (* _Amiram Eldar_, May 07 2021 *)
%o A325906 (PARI) {a(n) = ((-1)^n*sum(k=0, n-2, (-1)^k*10^2^k)+10^2^(n-1)-((-1)^n+3)/2)/9}
%Y A325906 Cf. A325907, A325910.
%K A325906 nonn
%O A325906 1,2
%A A325906 _Seiichi Manyama_, Sep 08 2019