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 A204425 #5 Mar 30 2012 18:58:08
%S A204425 1,2,0,0,1,2,1,2,0,1,2,0,1,2,0,0,1,2,0,1,2,1,2,0,1,2,0,1,2,0,1,2,0,1,
%T A204425 2,0,0,1,2,0,1,2,0,1,2,1,2,0,1,2,0,1,2,0,1,2,0,1,2,0,1,2,0,1,2,0,0,1,
%U A204425 2,0,1,2,0,1,2,0,1,2,1,2,0,1,2,0,1,2,0,1,2,0,1,2,0,1,2,0,1,2,0
%N A204425 Infinite matrix: f(i,j)=(2i+j+1 mod 3), by antidiagonals.
%C A204425 A block matrix over {0,1,2}. See A204263 for a guide to related matrices and permanents.
%e A204425 Northwest corner:
%e A204425 1 2 0 1 2 0
%e A204425 0 1 2 0 1 2
%e A204425 2 0 1 2 0 1
%e A204425 1 2 0 1 2 0
%e A204425 0 1 2 0 1 2
%e A204425 2 0 1 2 0 1
%t A204425 f[i_, j_] := Mod[2 i + j + 1, 3];
%t A204425 m[n_] := Table[f[i, j], {i, 1, n}, {j, 1, n}]
%t A204425 TableForm[m[8]] (* 8x8 principal submatrix *)
%t A204425 Flatten[Table[f[i, n + 1 - i],
%t A204425 {n, 1, 14}, {i, 1, n}]] (* A204425 *)
%t A204425 Permanent[m_] :=
%t A204425 With[{a = Array[x, Length[m]]},
%t A204425 Coefficient[Times @@ (m.a), Times @@ a]];
%t A204425 Table[Permanent[m[n]], {n, 1, 22}] (* A204426 *)
%Y A204425 Cf. A204426, A204263.
%K A204425 nonn,tabl
%O A204425 1,2
%A A204425 _Clark Kimberling_, Jan 15 2012