\nonstopmode
\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\pagestyle{empty}
\begin{document}
$$
f(x)={\begin{cases}1&-1\leq x<0\\{\frac  {1}{2}}&x=0\\1-x^{2}&0<x\leq 1\end{cases}}
$$
\end{document}
