Using \boxed{} in \newenvironment{envname}{here}{and here}

Question

There's a type of environment (shown below) which I'm using very frequently. I would like to define a new environment to begin it and end it with single commands.

\begin{equation}\boxed{\boxed{
\begin{gathered}
    i(t)=\frac{V_s}{R}\left[1-e^{-\frac{R}{L}t}\right]\\
    \textnormal{Time Constant (} \tau_L \textnormal{)} =\frac{L}{R}
\end{gathered}
}}\end{equation}

However, it seems *\boxed{}* cannot be split between the beginning-and-ending braces of \newenvironment{} \newenvironment{envname}{ *\boxed{* }{ *}* } (the parts enclosed in the asterisks have bad syntax).

Splitting two instances of "\boxed{}" is even worse syntactically.

For reference, this is the output I want:

I have to type out these formatted equations very frequently, and any way to condense the necessary commands is appreciated... Is there a way to get around the syntactical issues and define a \newenvironment{name}{beginning}{ending} properly? If possible, I would like to avoid using extra packages.

Sorry if any part of this post seems stupid. I'm new to StackExchange...

Answer 1

Of course you cannot have unbalanced braces in the begin/end code. A way out is to use \boxes:

\documentclass[twocolumn]{article}

\usepackage{amsmath}

\newenvironment{boxedeq}{%
   \equation\setbox0=\hbox\bgroup$\gathered
  }{%
   \endgathered$\egroup
   \fbox{\fbox{\box0}}%
   \endequation
}

\begin{document}

\begin{boxedeq}
i(t)=\frac{V_s}{R}\left[1-e^{-\frac{R}{L}t}\right] \\
\textnormal{Time Constant ($\tau_L$)} =\frac{L}{R}
\end{boxedeq}

\end{document}

Answer 2

With a “save box”:

\documentclass{article}
\usepackage{amsmath}

\newsavebox{\doubleboxedbox}
\newenvironment{doubleboxed}
 {\begin{lrbox}{\doubleboxedbox}$\displaystyle}
 {$\end{lrbox}\boxed{\boxed{\usebox{\doubleboxedbox}}}}

\begin{document}

\begin{equation}
\begin{doubleboxed}
  \begin{gathered}
    i(t)=\frac{V_s}{R}\left[1-e^{-\frac{R}{L}t}\right]\\
    \text{Time Constant ($\tau_L$)}=\frac{L}{R}
  \end{gathered}
\end{doubleboxed}
\end{equation}

\end{document}

(I simplified the input for the second line).

With expl3 facilities:

\documentclass{article}
\usepackage{amsmath}

\NewDocumentEnvironment{doubleboxed}{b}{\boxed{\boxed{#1}}}{}

\begin{document}

\begin{equation}
\begin{doubleboxed}
  \begin{gathered}
    i(t)=\frac{V_s}{R}\left[1-e^{-\frac{R}{L}t}\right]\\
    \text{Time Constant ($\tau_L$)}=\frac{L}{R}
  \end{gathered}
\end{doubleboxed}
\end{equation}

\end{document}

Answer 3

Another solution, using a patched version of the \doublebox command from fancybox:

\documentclass{article}
\usepackage{amsmath} 
\usepackage{fancybox} 
\usepackage{xpatch}
\makeatletter
\xpatchcmd{\@doublebox}{\advance\fboxsep .5pt}{\advance\fboxsep 2.5pt}{}{}
\newenvironment{boxedequation}{\equation\doublebox\bgroup$\gathered}
{\endgathered$\egroup\endequation}
\makeatother

\begin{document}

\begin{boxedequation}
    i(t)=\frac{V_s}{R}\left[1-e^{-\frac{R}{L}t}\right]\\
    \textnormal{Time Constant (} \tau_L \textnormal{)} =\frac{L}{R}
\end{boxedequation}

\end{document}