Alternative to aligned environment for enumerate

Question

I'm using the aligned environment within an itemize environment. I need the aligned environment so the label for the \item will be on the same line as the equation.

For example:

\begin{enumerate}
\item $\begin{aligned}[t]
%Big equation with many lines over two or more pages
\end{aligned}$
\end{enumerate}

However, I need to have this same label and equation alignment (i.e., on the same line), but also \allowdisplaybreaks for the huge equation within the aligned environment.

The problem as I understand it is that the aligned environment creates an unbreakable box around the multi-line equation. However, if I use the align environment, then the equation will not be on the same line as the \item label.

Ideas?

EDIT: Here's an MWE

\documentclass{article}
\usepackage{amsmath}
\usepackage{enumitem}

\allowdisplaybreaks[1]
\setitemize{topsep=5pt}

\begin{document}
 \begin{enumerate}
  \item Equation 1\\
  \item Equation 2\\
  \item $\begin{aligned}[t]
   line 1&\\%This equation will span 2+ pages
   &line2\\
  \end{aligned}$
 \end{enumerate}
\end{document}

In this example, line 1 is on the same line as the label (3.), but if this code were placed in the middle of a page, say, then the item in the aligned environment would start on the next page, and I need it to start right under the 2nd item.

Answer 1

It isn't clear that you can't do something more like:

\documentclass{article}
\usepackage[leqno]{amsmath}
\usepackage{enumitem}

\allowdisplaybreaks[1]
\setitemize{topsep=5pt}

\begin{document}

\begin{align}
  Equation 1\\
  Equation 2\\
   line 1&\\%This equation will span 2+ pages
   &line2 \nonumber\\
   &line2 \nonumber\\
   &line2 \nonumber\\
   &line2 \nonumber\\
   &line2 \nonumber\\
   &line2 \nonumber\\
   &line2 \nonumber\\
   &line2 \nonumber
  \end{align}
\end{document}

Answer 2

One way to do this would be to pause the enumerate environment, and resume the list after the equation:

Notes:

• The paperheight=6.1cm,paperwidth=9cm were adjustments made to make the image shown here illustrate that the desired result was achieved.

Code:

\documentclass{article}
\usepackage[paperheight=5.6cm,paperwidth=9cm, showframe]{geometry}
\usepackage{amsmath}
\usepackage{enumitem}
\newcounter{TempCounter}
\newcommand{\PauseListEnvironment}{%
    \stepcounter{enumi}%
    \setcounter{TempCounter}{\value{enumi}}%
}%
\newcommand{\ResumeList}{%
    \makebox[\dimexpr\leftmargin\relax][r]{\arabic{TempCounter}.\hspace{\labelsep}}%
    \belowdisplayskip=0pt%
}%
\allowdisplaybreaks[1]
\setitemize{topsep=5pt}
\begin{document}
 \begin{enumerate}[series=xxx]
  \item Equation 1
  \item Equation 2
  \item Equation 3
  \PauseListEnvironment%
 \end{enumerate}
 \begingroup
     \abovedisplayskip=0pt%
     \belowdisplayskip=0pt%
     \begin{flalign}\ResumeList
           F &= ma &\
           G &= mbc\
           E &= mc^2 \
           H &= md^3 \
           I &= md^4
      \end{flalign}%
  \endgroup%
 \begin{enumerate}[resume=xxx]
  \item Equation 5
 \end{enumerate}
\end{document}

Answer 3

The following MWE creates a mathlist that functions similar to an aligned environment:

\documentclass{article}
\usepackage[paper=a6paper]{geometry}% Just for this example
\usepackage{environ,enumitem}% http://ctan.org/pkg/environ
\makeatletter
\let\olditem\item% Store regular \item macro
\newlength{\LHS}
\def\processitem#1&#2\relax{%
  \olditem\hspace*{\LHS}%
  \llap{$\displaystyle #1{}$}%
  $\displaystyle{}#2$%
}
\NewEnviron{mathlist}[1][3em]{%
  \setlength{\LHS}{#1}% Set Left-Hand Side length
  \g@addto@macro{\BODY}{\item\relax\item}% Used to delimit the items; last item identified by \item\relax\item
  \def\item##1\item{% Redefine \item to capture contents
    \def\optarg{##1}%
    \expandafter\ifx\optarg\relax\else% Last item not reached
      \expandafter\processitem##1\relax% Process item
      \expandafter\item% Recursively continue processing items
    \fi
  }
  \begin{itemize}[label={},labelsep=0pt]
    \BODY% Process environment (save items)
  \end{itemize}
}
\makeatother
\begin{document}
\begin{enumerate}
  \item Equation 1
  \item Equation 2
  \item
    \begin{mathlist}[1em]
      \item F &= ma
      \item G &= mbc
      \item E &= mc^2
      \item H &= md^3
      \item I &= md^4
      \item F &= ma
      \item G &= mbc
      \item E &= mc^2
      \item H &= md^3
      \item I &= md^4
      \item F &= ma
      \item G &= mbc
      \item E &= mc^2
      \item H &= md^3
      \item I &= md^4
      \item F &= ma
      \item G &= mbc
      \item E &= mc^2
      \item H &= md^3
      \item I &= md^4
    \end{mathlist}
  \item Equation 3
  \item Equation 4
\end{enumerate}
\end{document}

\items inside mathlist are processed in a delimited requiring the use of & (it would be possible to rewrite it to work with/without).

mathlist takes a single optional argument, specifying the width of the left-hand side of equation set (default is 2em), although there are other horizontal lengths at play since you're in a list (itemize with label = {}, to be specific). mathlist uses environ in order to grab the entire environment contents, and then processes each item separately. This technique is taken from Order items in enumerate environment automatically.

In each \item inside mathlist, the math content is forced to be in \displaystyle.

Answer 4

TABstacks may be the thing to assist you in this regard. They can have align or tabular like syntax, and only take up as much width as needed. Furthermore, by using understacks, the first line of equation will be aligned with the enumerator index.

As Peter Grill reminds me, any single TABstack will not break across a page boundary, because it is boxed material.

\documentclass{article}
\usepackage{tabstackengine}
\def\dfrac{\displaystyle\frac}
\stackMath
\strutlongstacks{T}
\setstacktabulargap{0pt}
\setstackgap{S}{6pt}
\begin{document}
\begin{enumerate}
\item \alignLongunderstack{
  5x + 2y =& 7\\
  2x  -2y =&0
}
\item \tabularShortunderstack{rrrl}{
\dfrac{1}{2}x^2 +& 3x -& 7 =& 1\\
 7 x^2 -& \dfrac{3}{4} x +& 12 =& 17\\
 5 x^2 +& \dfrac{2}{3} x +& 5 =& 9
}
\end{enumerate}
\end{document}