Rectangular framing some relations

Question

How can I create a rectangular frame around some relations?

Answer 1

I suggest you place an aligned environment inside a \boxed directive.

\documentclass{article} % or some other suitable document class
\usepackage{amsmath} % for 'aligned' env. and '\boxed' and '\text' commands
\usepackage{newtxtext,newtxmath} % optional (Times Roman fonts)
\newcommand\R{\mathbb{R}} % handy shortcut command
\begin{document}
[
\boxed{\begin{aligned}
\text{function $f\colon\R\to\R$}
   &\Leftrightarrow 
   \text{linear transformation $T_A\colon\R^n\to\R^m$} \
\text{input $x\in\R$} 
   &\Leftrightarrow
   \text{input $\vec{x}\in\R^m$} \
\cdots &\Leftrightarrow \cdots \
\text{range of $f$}
   &\Leftrightarrow
   \text{$\mathcal{C}(A)\equiv{}$column space of $A={}$range of $T_A$}
\end{aligned}}
]
\end{document}