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 A204427 #7 Mar 06 2017 00:08:43
%S A204427 2,0,1,1,2,0,2,0,1,2,0,1,2,0,1,1,2,0,1,2,0,2,0,1,2,0,1,2,0,1,2,0,1,2,
%T A204427 0,1,1,2,0,1,2,0,1,2,0,2,0,1,2,0,1,2,0,1,2,0,1,2,0,1,2,0,1,2,0,1,1,2,
%U A204427 0,1,2,0,1,2,0,1,2,0,2,0,1,2,0,1,2,0,1,2,0,1,2,0,1,2,0,1,2,0,1
%N A204427 Infinite matrix: f(i,j)=(2i+j+2 mod 3), read by antidiagonals.
%C A204427 A block matrix over {0,1,2}. See A204263 for a guide to related matrices and permanents.
%e A204427 Northwest corner:
%e A204427 2 0 1 2 0 1
%e A204427 1 2 0 1 2 0
%e A204427 0 1 2 0 1 2
%e A204427 2 0 1 2 0 1
%e A204427 1 2 0 1 2 0
%e A204427 0 1 2 0 1 2
%t A204427 f[i_, j_] := Mod[2 i + j + 2, 3];
%t A204427 m[n_] := Table[f[i, j], {i, 1, n}, {j, 1, n}]
%t A204427 TableForm[m[8]] (* 8x8 principal submatrix *)
%t A204427 Flatten[Table[f[i, n + 1 - i],
%t A204427 {n, 1, 14}, {i, 1, n}]] (* A204427 *)
%t A204427 Permanent[m_] :=
%t A204427 With[{a = Array[x, Length[m]]},
%t A204427 Coefficient[Times @@ (m.a), Times @@ a]];
%t A204427 Table[Permanent[m[n]], {n, 1, 22}] (* A204428 *)
%Y A204427 Cf. A204428, A204263.
%K A204427 nonn,tabl
%O A204427 1,1
%A A204427 _Clark Kimberling_, Jan 15 2012