\[P_{mn}(\mathbf{y})={(t_{n}(\mathbf{x}(\mathbf{y})))}^{K+1}+\sum_{l=m+2}^{K}%
\frac{l}{K+2}x_{l}(\mathbf{y}){(t_{n}(\mathbf{x}(\mathbf{y})))}^{l-1},\]