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 A204431 #5 Mar 30 2012 18:58:08
%S A204431 2,1,1,0,0,0,2,2,2,2,1,1,1,1,1,0,0,0,0,0,0,2,2,2,2,2,2,2,1,1,1,1,1,1,
%T A204431 1,1,0,0,0,0,0,0,0,0,0,2,2,2,2,2,2,2,2,2,2,1,1,1,1,1,1,1,1,1,1,1,0,0,
%U A204431 0,0,0,0,0,0,0,0,0,0,2,2,2,2,2,2,2,2,2,2,2,2,2,1,1,1,1,1,1,1,1
%N A204431 Symmetric matrix: f(i,j)=(2i+j+1 mod 3), by antidiagonals.
%C A204431 A block matrix over {0,1,2}. See A204263 for a guide to related matrices and permanents.
%e A204431 Northwest corner:
%e A204431 2 1 0 2 1 0
%e A204431 1 0 2 1 0 2
%e A204431 0 2 1 0 2 1
%e A204431 2 1 0 2 1 0
%e A204431 1 0 2 1 0 2
%e A204431 0 2 1 0 2 1
%t A204431 f[i_, j_] := Mod[2 i + 2 j + 1, 3];
%t A204431 m[n_] := Table[f[i, j], {i, 1, n}, {j, 1, n}]
%t A204431 TableForm[m[8]] (* 8x8 principal submatrix *)
%t A204431 Flatten[Table[f[i, n + 1 - i],
%t A204431 {n, 1, 14}, {i, 1, n}]] (* A204431 *)
%t A204431 Permanent[m_] :=
%t A204431 With[{a = Array[x, Length[m]]},
%t A204431 Coefficient[Times @@ (m.a), Times @@ a]];
%t A204431 Table[Permanent[m[n]], {n, 1, 22}] (* A204432 *)
%Y A204431 Cf. A204431, A204263.
%K A204431 nonn,tabl
%O A204431 1,1
%A A204431 _Clark Kimberling_, Jan 15 2012