Environments nested in Lists

Question

I have a list with several items consisting solely of an align environment:

\begin{enumerate}
\item
\begin{align}
e^{\pi i} + 1 &= 0\\
1 + 1 &= 2
\end{align}
\end{enumerate}

By default, the environment begins on the line after the numbering. Other list environments with nested environments also prepend a line break automatically:

\begin{itemize}
\item
\[e^{\pi i} + 1 = 0\]
\end{itemize}

What is the best method to start the environment on the same line as the numbering?

Answer 1

If you don't need to number the equations (and this wouldn't make much sense anyway), then aligned is what you're looking for:

\begin{enumerate}
\item
$\begin{aligned}[t]
e^{\pi i} + 1 &= 0\\
1 + 1 &= 2
\end{aligned}$
\end{enumerate}

The environments aligned, alignedat and gathered can take an optional argument telling LaTeX what vertical alignment we want with respect to the context; it can be [t] or [b] (default is center alignment), similarly to tabular, array and minipage.

Answer 2

I think you do not want really to put an equation into enumerate. What you want is to put equation number on left. Here is how:

\documentclass[leqno]{article}
\begin{document}
\begin{equation}
  e^{\pi i} + 1 = 0
\end{equation}
\end{document}

However, if you really, really want to use enumerate, try this:

\begin{enumerate}
\item\leavevmode\par\vspace*{-25pt}%
  \[e^{\pi i} + 1 = 0\]
\end{enumerate}

Answer 3

A simple method is to use an in-text equation, with \displaystyle if necessary.

\documentclass{article}
\begin{document}
\begin{itemize}
\item This item contains text.
\item \( \displaystyle \sum_{j=0}^\infty \frac{1}{j^2} = \frac{\pi^2}{6}. \)
\end{itemize}
\end{document}