\[\begin{aligned}
f\left(x,y\right)=&p_1 x^n y^m+p_2 x^{\left(n-1\right)} y^m+\ldots+p_{n+1} y^m+\ldots \\
&p_{n+2}x^ny^{\left(m-1\right)}+p_{n+3}x^{\left(n-1\right)}y^{\left(m-1\right)}+\ldots+p_{2\left(n+1\right)}y^{\left(m-1\right)}+\ldots \\
&\ldots \\
&p_{m\left(n+1\right)+1}*x^n+p_{m\left(n+1\right)+2}*x^{\left(n-1\right)}+\ldots+p_{\left(n+1\right)\left(m+1\right)}
\end{aligned} \]
<script src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML" type="text/javascript"></script>