Create a new math mode

Question

Motivation

When I use math mode inside of itemize environments for multiline equations, I have to do something like the following

...
\begin{itemize}
  \item $\begin{aligned}[t]
    equation stuff
  \end{aligned}$
  ...
\end{itemize}
...

Question

How can I wrap the two components $\begin{aligned}[t] and \end{aligned}$ into a new symbol, e.g. §? I'd like to write the above example as

...
\begin{itemize}
  \item §equation stuff§
  ...
\end{itemize}
...

It is favorable for the solution to be robust.

---

(It is not required for the new symbol to be §.)

Answer 1

The \catcode`§=\active method won't work if the document declares

\usepackage[utf8]{inputenc}

because § is a two byte character in UTF-8. For this case you can use

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}

\usepackage{newunicodechar}
\newunicodechar{§}{\mymath}
\def\mymath#1§{$\begin{aligned}[t] #1\end{aligned}$}

\newenvironment{itemalign}
  {$\aligned[t]}
  {\endaligned$}

\begin{document}
\begin{itemize}
\item §x&=2\\y&=6§
\item \begin{itemalign}x&=2\\y&=6\end{itemalign}
\end{itemize}
\end{document}

that has the advantage of working also with LuaLaTeX and XeLaTeX (where the call to inputenc should not be present).

However, I'd recommend using the new environment method.

Answer 2

If I were you, I would use two different macros to open and close the environment. I would do something like

\def\ba#1\ea{$\begin{aligned}[t]#1\end{aligned}$}

that can be use like that

\begin{itemize}
\item \ba equation stuff \ea
\end{itemize}

Answer 3

How about making § active?

\documentclass{article}

\usepackage{amsmath}

\catcode`§=\active
\def§#1§{$\begin{aligned}[t] #1\end{aligned}$}

\begin{document}
\begin{itemize}
\item §x&=2\\y&=6§
\end{itemize}
\end{document}

Then we can scan an argument up to the next occurence of § and wrap this in aligned.

---

As @egreg pointed out, this will be problematic using pdflatex and inputenc (see also: Catcodes of unicode characters with \usepackage[utf8]{inputenc}). A simple workaround in this case is changing § to e.g. |.