A Conditional Defintion

$$f(x,y)= \left\{ \begin{array}{ll} \frac{x^2y}{x^4+y^2} & \m... ...\ 0 & \mbox{if $(x,y) = (0,0)$} \\ \end{array} \right. \\$$

                \[
                f(x,y)= \left\{ \begin{array}{ll}
                			\frac{x^2y}{x^4+y^2} & \mbox{if $(x,y) \neq (0,0)$} \\
                			0 & \mbox{if $(x,y) = (0,0)$} \\
                			  \end{array}
                		 \right. \\
                \]

The $\backslash$right. command closes the left delimiter $\backslash\{$ syntactically without displaying anything.