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 A131406 #7 Nov 17 2016 08:03:03
%S A131406 1,2,1,1,2,1,2,1,2,1,1,2,1,2,1,2,1,2,1,2,1,1,2,1,2,1,2,1,2,1,2,1,2,1,
%T A131406 2,1,1,2,1,2,1,2,1,2,1,2,1,2,1,2,1,2,1,2,1
%N A131406 3*A128174 - 2*A000012(signed).
%C A131406 Row sums = A032766, congruent to {0,1} mod 3: (1, 3, 4, 6, 7, 9, 10,...).
%C A131406 Sequence array for the expansion of (1+2x)/(1-x^2). A105476 is an eigensequence. [From _Paul Barry_, Nov 03 2010]
%F A131406 3*A128174 - 2*A000012(signed + - + 1 by columns). (1, 2, 1, 2, 1,...) in every column.
%F A131406 Triangle T(n,k)=if(k<=n,(3-(-1)^(n-k))/2). [From _Paul Barry_, Nov 03 2010]
%e A131406 First few rows of the triangle are:
%e A131406 1;
%e A131406 2, 1;
%e A131406 1, 2, 1;
%e A131406 2, 1, 2, 1;
%e A131406 1, 2, 1, 2, 1;
%e A131406 2, 1, 2, 1, 2, 1;
%e A131406 ...
%t A131406 T[n_, k_] := Mod[n-k, 2]+1; Table[T[n, k], {n, 1, 10}, {k, 1, n}] // Flatten (* _Jean-François Alcover_, Nov 17 2016 *)
%Y A131406 Cf. A128174, A000012, A032766.
%K A131406 nonn,tabl
%O A131406 1,2
%A A131406 _Gary W. Adamson_, Jul 07 2007