\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}

\begin{document}
\[
  \varphi(r) = 
    \begin{cases}
       1 & r \le R_1 \\
       c_0 + c_2r^2 + c_4r^4 + c_6r^6  & r \in (R_1, R_2) \\
       0 & r \ge R_2
  \end{cases}
\]

\end{document}
