p(x,y)=\sum_{\tiny{\begin{array}{c}i<k\\j<l\end{array}}}f_{i,j}x^iy^j+\sum_{\tiny{\begin{array}{c}i\geq k\\j\leq n-i\end{array}}}f_{i,j}g_i(x)y^j+\sum_{\tiny{\begin{array}{c}j\geq l\\i\leq n-j\end{array}}}f_{i,j}x^ih_j(y)\quad(*)