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 A204429 #13 Oct 25 2021 16:41:46
%S A204429 1,0,0,2,2,2,1,1,1,1,0,0,0,0,0,2,2,2,2,2,2,1,1,1,1,1,1,1,0,0,0,0,0,0,
%T A204429 0,0,2,2,2,2,2,2,2,2,2,1,1,1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,2,2,
%U A204429 2,2,2,2,2,2,2,2,2,2,1,1,1,1,1,1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0
%N A204429 Symmetric matrix: f(i,j)=(2*i + 2*j) mod 3, by antidiagonals.
%C A204429 A block matrix over {0,1,2}. See A204263 for a guide to related matrices and permanents.
%e A204429 Northwest corner:
%e A204429 1 0 2 1 0 2
%e A204429 0 2 1 0 2 1
%e A204429 2 1 0 2 1 0
%e A204429 1 0 2 1 0 2
%e A204429 0 2 1 0 2 1
%e A204429 2 1 0 2 1 0
%t A204429 f[i_, j_] := Mod[2 i + 2 j, 3]; (* symmetric *)
%t A204429 m[n_] := Table[f[i, j], {i, 1, n}, {j, 1, n}]
%t A204429 TableForm[m[8]] (* 8x8 principal submatrix *)
%t A204429 Flatten[Table[f[i, n + 1 - i], {n, 1, 14}, {i, 1, n}]]
%Y A204429 Cf. A204430, A204263.
%K A204429 nonn,tabl
%O A204429 1,4
%A A204429 _Clark Kimberling_, Jan 15 2012
%E A204429 Definition corrected by _Georg Fischer_, Oct 25 2021