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 A203955 #6 Jul 12 2012 00:39:54
%S A203955 1,2,2,3,5,3,1,8,8,1,2,5,14,5,2,3,5,11,11,5,3,1,8,11,15,11,8,1,2,5,14,
%T A203955 13,13,14,5,2,3,5,11,14,19,14,11,5,3,1,8,11,15,19,19,15,11,8,1,2,5,14,
%U A203955 13,16,28,16,13,14,5,2,3,5,11,14,19,22,22,19,14,11,5,3,1,8
%N A203955 Symmetric matrix based on (1,2,3,1,2,3,1,2,3...), by antidiagonals.
%C A203955 Let s be the periodic sequence (1,2,3,1,2,3,...) and let T be the infinite square matrix whose n-th row is formed by putting n-1 zeros before the terms of s. Let T' be the transpose of T. Then A203955 represents the matrix product M=T'*T. M is the self-fusion matrix of s, as defined at A193722. See A203956 for characteristic polynomials of principal submatrices of M, with interlacing zeros.
%e A203955 Northwest corner:
%e A203955 1....2....3....1....2....3
%e A203955 2....5....8....5....5....8
%e A203955 3....8....14...11...11...14
%e A203955 1....5....11...15...13...14
%t A203955 t = {1, 2, 3}; t1 = Flatten[{t, t, t, t, t, t, t, t, t}];
%t A203955 s[k_] := t1[[k]];
%t A203955 U = NestList[Most[Prepend[#, 0]] &, #, Length[#] - 1] &[
%t A203955 Table[s[k], {k, 1, 15}]];
%t A203955 L = Transpose[U]; M = L.U; TableForm[M] (* A203955 *)
%t A203955 m[i_, j_] := M[[i]][[j]];
%t A203955 Flatten[Table[m[i, n + 1 - i], {n, 1, 12}, {i, 1, n}]]
%Y A203955 Cf. A203956, A202453.
%K A203955 nonn,tabl
%O A203955 1,2
%A A203955 _Clark Kimberling_, Jan 08 2012