[Tex/LaTex] Square bracket math mode

Question

I need to change the two braces to two square brackets that are surrounding cos and sin in both equations.

\documentclass[oneside,10pt]{memoir}
\usepackage[T1]{fontenc}

\usepackage{amsmath}
\usepackage{siunitx}

\newcommand*{\bfrac}[2]{\genfrac{\lbrace}{\rbrace}{0pt}{}{#1}{#2}}

\begin{document}
\begin{equation}
\Psi(r,\phi) =
\begin{cases}
\frac{A}{J_l (U)} J_l(\frac{U \cdot  r}{a}) \bfrac{\cos(l\phi)}{\sin(l\phi)}; & r < a \\
\\
\frac{A}{K_l (U)} K_l(\frac{W \cdot  r}{a})\bfrac{\cos(l\phi)}{\sin(l\phi)};  &  r > a
\end{cases}
\end{equation}
\end{document}

Answer 1

You only need to modify your \bfrac command.

\documentclass[oneside,10pt]{memoir}
\usepackage[T1]{fontenc}

\usepackage{amsmath}
\usepackage{siunitx}

\newcommand*{\bfrac}[2]{\genfrac{\lbrace}{\rbrace}{0pt}{}{#1}{#2}}
\newcommand*{\Bfrac}[2]{\genfrac{[}{]}{0pt}{}{#1}{#2}}

\begin{document}
\begin{equation}
\Psi(r,\phi) =
\begin{cases}
\frac{A}{J_l (U)} J_l(\frac{U \cdot  r}{a}) \bfrac{\cos(l\phi)}{\sin(l\phi)}; & r < a \\
\\
\frac{A}{K_l (U)} K_l(\frac{W \cdot  r}{a})\Bfrac{\cos(l\phi)}{\sin(l\phi)};  &  r > a
\end{cases}
\end{equation}
\end{document}