Custom environment with minipage and align*

Question

I would like to have an environment that puts two equations side by side. The following works but Is there a way to make this a dedicated newenvironment?

  \documentclass[10pt]{article}
    \begin{document}
    \begin{minipage}{0.5\linewidth}
      \begin{equation}
        y_1(x)=x^2
      \end{equation}
    \end{minipage}
    \hspace{0.5cm}
    \begin{minipage}{0.5\linewidth}
      \begin{equation}
        y_2(x)=2x+1
      \end{equation}
    \end{minipage}
  \end{document}

My attempt is the folowing:

\newenvironment{sidebyside}[1]
{%
  \begin{minipage}{#1\linewidth}
  \csname align*\endcsname
}{%
  \csname endalign*\endcsname
  \end{minipage}
}

Which gives the error:

LaTeX Error: \begin{minipage} on input line ... ended by \end{sidebyside}.

Answer 1

\begin{minipage}, \end{minipage} won't work in the definition of the environment and will trigger an error

\begin{minipage} on input line 16 ended by \end{sidebyside}.

due to the way align* is processed (it reads the content more than once for calculating the alignments); when this internal processing finishes, the \begin{minipage} will incorrectly be matched to the \end{sidebyside} and the error occurs. If you use equation instead of align, then you can use \begin{minipage}, \endminipage, for example.

Here's one possibility, using \minipage, \end{minipage} instead:

\documentclass[10pt]{article}
\usepackage{amsmath}

\newenvironment{sidebyside}[1]
{%
  \minipage{#1\linewidth}
  \csname align*\endcsname
}{%
  \csname endalign*\endcsname
  \endminipage
}

\begin{document}

\noindent
\begin{sidebyside}{0.5}
  y_1(x)=x^2
\end{sidebyside}%
\begin{sidebyside}{0.5}
  y_2(x)=2x+1
\end{sidebyside}

\end{document}

Answer 2

You can use environ package and lead a simple life ;)

\documentclass[10pt]{article}
\usepackage{environ}
\NewEnviron{sidebyside}[1]
{%
  \begin{minipage}{#1\linewidth}
  \begin{equation}
  \BODY
  \end{equation}
  \end{minipage}%
}
\begin{document}

\noindent
\begin{sidebyside}{0.5}
  y_1(x)=x^2
\end{sidebyside}%
\begin{sidebyside}{0.5}
  y_2(x)=2x+1
\end{sidebyside}

\end{document}

One can make the width option optional too.

\documentclass[10pt]{article}
\usepackage{environ}
\NewEnviron{sidebyside}[1][0.5]
{%
  \begin{minipage}{#1\linewidth}
  \begin{equation}
  \BODY
  \end{equation}
  \end{minipage}%
}
\begin{document}

\noindent
\begin{sidebyside}
  y_1(x)=x^2
\end{sidebyside}%
\begin{sidebyside}
  y_2(x)=2x+1
\end{sidebyside}

\end{document}

so that \begin{sidebyside} takes 0.5\linewidth unless you specify it like \begin{sidebyside}[0.4]

Answer 3

Maybe a simpler syntax will help you:

\documentclass{article}
\usepackage{amsmath}
\usepackage{lipsum} % just for the example

\newlength{\sbswidth}
\newenvironment{sidebysideeq}[1][0.5]
 {\[% start equation
  \setlength{\sbswidth}{#1\linewidth}%
  \minipage{\sbswidth}
  \centering
  $\!\aligned
 }
 {\endaligned$\endminipage\]}
\newcommand{\breaksideeq}{%
  \endaligned$\endminipage
  \setlength{\sbswidth}{\dimexpr\linewidth-\sbswidth}%
  \minipage{\sbswidth}
  \centering
  $\!\aligned
}

\begin{document}

\lipsum*[2]
\begin{sidebysideeq}
  y_1(x)=x^2
\breaksideeq
  y_2(x)=2x+1
\end{sidebysideeq}
\lipsum*[3]
\begin{sidebysideeq}[0.3]
  y_1(x)&=x^2\\
  y_2(x)&=x^3-x
\breaksideeq
  y_3(x)=2x+1+a+b+c+d+e+f+g
\end{sidebysideeq}
\lipsum[2]
\end{document}

The optional argument to sidebysideeq tells the fraction of \linewidth reserved for the left part, default is 0.5.

One can easily add a key-value interface for setting more aspects of the environment: here I use a key for the left fraction and for the reciprocal vertical alignment. The default values are equivalent to left=0.5 and align=c. The possible values for align are c, t or b (but the last one is probably not really useful).

\documentclass{article}
\usepackage{amsmath,keyval}
\usepackage{lipsum} % just for the example

\makeatletter
\define@key{SBS}{left}{\def\SBS@left{#1}}
\define@key{SBS}{align}{\def\SBS@align{#1}}
\def\SBS@left{0.5}
\def\SBS@align{c}

\newlength{\sbswidth}
\newenvironment{sidebysideeq}[1][]
 {\[% start equation
  \setkeys{SBS}{#1}%
  \setlength{\sbswidth}{\SBS@left\linewidth}%
  \minipage[\SBS@align]{\sbswidth}
  \centering
  $\!\aligned[\SBS@align]
 }
 {\endaligned$\endminipage\]}
\newcommand{\breaksideeq}{%
  \endaligned$\endminipage
  \setlength{\sbswidth}{\dimexpr\linewidth-\sbswidth}%
  \minipage[\SBS@align]{\sbswidth}
  \centering
  $\!\aligned[\SBS@align]
}
\makeatother

\begin{document}
\lipsum*[2]
\begin{sidebysideeq}
  y_1(x)=x^2
\breaksideeq
  y_2(x)=2x+1
\end{sidebysideeq}
\lipsum*[3]
\begin{sidebysideeq}[left=0.3,align=t]
  y_1(x)&=x^2\\
  y_2(x)&=x^3-x
\breaksideeq
  y_3(x)=2x+1+a+b+c+d+e+f+g
\end{sidebysideeq}
\lipsum[2]
\end{document}

If you want to support \tag, then a more complex approach should be taken.

\documentclass{article}
\usepackage{amsmath,keyval,environ}
\usepackage{lipsum} % just for the example

\makeatletter
\define@key{SBS}{left}{\def\SBS@left{#1}}
\define@key{SBS}{align}{\def\SBS@align{#1}}
\def\SBS@left{0.5}
\def\SBS@align{c}

\newlength{\sbswidth}
\NewEnviron{sidebysideeq}[1][]{%
  \par\centering
  \setkeys{SBS}{#1}%
  \penalty\predisplaypenalty
  \if\SBS@align t\vskip-\baselineskip\vskip\prevdepth\fi
  \setlength{\sbswidth}{\SBS@left\linewidth}%
  \expandafter\make@sidebysideeq\BODY\@nil
}

\def\make@sidebysideeq#1\breaksideeq#2\@nil{%
  \begin{minipage}[\SBS@align]{\sbswidth}
  \vskip-\baselineskip\vskip\abovedisplayskip
  \begin{align*}#1\end{align*}
  \vskip\belowdisplayskip
  \end{minipage}%
  \setlength{\sbswidth}{\dimexpr\linewidth-\sbswidth}%
  \begin{minipage}[\SBS@align]{\sbswidth}
  \vskip-\baselineskip\vskip\abovedisplayskip
  \begin{align*}#2\end{align*}
  \vskip\belowdisplayskip
  \end{minipage}
}
\makeatother


\begin{document}
\lipsum*[2]
\begin{sidebysideeq}[align=t]
  y_1(x)=x^2
\breaksideeq
  y_2(x)=2x+1
\end{sidebysideeq}
\lipsum*[3]
\begin{sidebysideeq}[left=0.3,align=t]
  y_1(x)&=x^2 \tag{*}\\
  y_2(x)&=x^3-x
\breaksideeq
  y_3(x)=2x+1+a+b+c+d+e+f+g
\end{sidebysideeq}
\lipsum[2]
\end{document}