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 A204433 #14 Oct 25 2021 14:20:23
%S A204433 0,2,2,1,1,1,0,0,0,0,2,2,2,2,2,1,1,1,1,1,1,0,0,0,0,0,0,0,2,2,2,2,2,2,
%T A204433 2,2,1,1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,2,2,2,2,2,2,2,2,2,2,2,1,1,
%U A204433 1,1,1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0,2,2,2,2,2,2,2,2
%N A204433 Symmetric matrix: f(i,j) = (2*i + 2*j + 2) mod 3, by antidiagonals.
%C A204433 A block matrix over {0,1,2}. See A204263 for a guide to related matrices and permanents.
%e A204433 Northwest corner:
%e A204433 0 2 1 0 2 1
%e A204433 2 1 0 2 1 0
%e A204433 1 0 2 1 0 2
%e A204433 0 2 1 0 2 1
%e A204433 2 1 0 2 1 0
%e A204433 1 0 2 1 0 2
%t A204433 f[i_, j_] := Mod[2 i + 2 j + 2, 3];
%t A204433 m[n_] := Table[f[i, j], {i, 1, n}, {j, 1, n}]
%t A204433 TableForm[m[8]] (* 8x8 principal submatrix *)
%t A204433 Flatten[Table[f[i, n + 1 - i], {n, 1, 14}, {i, 1, n}]]
%Y A204433 Cf. A204432, A204434, A204263.
%K A204433 nonn,tabl
%O A204433 1,2
%A A204433 _Clark Kimberling_, Jan 15 2012
%E A204433 Definition corrected by _Georg Fischer_, Oct 25 2021