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 A204020 #12 Feb 21 2013 12:13:28
%S A204020 1,-1,3,-5,1,15,-31,14,-1,105,-247,157,-30,1,945,-2433,1892,-553,55,
%T A204020 -1,10395,-28653,25573,-9620,1554,-91,1,135135,-393279,388810,-173773,
%U A204020 37550,-3738,140,-1,2027025,-6169455
%N A204020 Array: row n shows the coefficients of the characteristic polynomial of the n-th principal submatrix of min(i,j)^2 (A106314).
%C A204020 Let p(n)=p(n,x) be the characteristic polynomial of the n-th principal submatrix. The zeros of p(n) are real, and they interlace the zeros of p(n+1). See A202605 and A204016 for guides to related sequences.
%C A204020 Constant term of p(n,x) is A001147(n), and the coefficient of the linear term is A000330(n). - _Enrique Pérez Herrero_, Feb 20 2013
%D A204020 (For references regarding interlacing roots, see A202605.)
%e A204020 Top of the array:
%e A204020 1.....-1
%e A204020 3.....-5.....1
%e A204020 15....-31....14....-1
%e A204020 105...-247...157...-30...1
%t A204020 f[i_, j_] := Min[i^2, j^2];
%t A204020 m[n_] := Table[f[i, j], {i, 1, n}, {j, 1, n}]
%t A204020 TableForm[m[6]] (* 6x6 principal submatrix *)
%t A204020 Flatten[Table[f[i, n + 1 - i],
%t A204020 {n, 1, 15}, {i, 1, n}]] (* A106314 *)
%t A204020 p[n_] := CharacteristicPolynomial[m[n], x];
%t A204020 c[n_] := CoefficientList[p[n], x]
%t A204020 TableForm[Flatten[Table[p[n], {n, 1, 10}]]]
%t A204020 Table[c[n], {n, 1, 12}]
%t A204020 Flatten[%] (* A204020 *)
%t A204020 TableForm[Table[c[n], {n, 1, 10}]]
%Y A204020 Cf. A106314, A202605, A204016.
%K A204020 tabl,sign
%O A204020 1,3
%A A204020 _Clark Kimberling_, Jan 11 2012