\[f(a+\lambda,b+\mu)=f+\left(\lambda\frac{\partial}{\partial x}+\mu\frac{%
\partial}{\partial y}\right)f+\dots+\frac{1}{n!}\left(\lambda\frac{\partial}{%
\partial x}+\mu\frac{\partial}{\partial y}\right)^{n}f+R_{n},\]