Creation of enumerate-like enviroment in gather and align environments

Question

I use the gather and align-aligned environments of amsmath package and I want to create a list inside them like the one created with the enumerate environment. How can I achieve that?

A MWE would be that:

\documentclass[12pt]{article}
\usepackage[top=0.3in, bottom=1.2in, left=0.8in, right=0.8in]{geometry}

\setlength{\parindent}{0cm}

\usepackage[fleqn]{amsmath}
\usepackage{unicode-math}

\everymath{\displaystyle}

\newcommand{\3}{\vspace{0.3cm}}

\begin{document}

\begin{gather*}
\begin{aligned}
&x+y=0\\
&y+z=0\\
&z+x=0
\end{aligned}
\end{gather*}

\end{document}

Answer 1

I would recommend using an aligned within an enumaerated list

otherwise, you should really use the equation numbering built into align:

---

Alternatively, you can use a custom macro such as \Item below which increments a counter and add is its value before the equation to emulate sort of a enumerate type of environment. I am recommending you not use this as I am sure there are numerous problems with it, even though it appears to work for the test case given.

---

Code: Recommended

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

\begin{document}
\begin{enumerate}
\item $\begin{aligned}[t]
        \sin^2 \theta + \cos^2 \theta &= 1 \\
        F &= ma 
      \end{aligned}$
\item $\begin{aligned}[t]
        E &= mc^2 \\
        \sin^2 \theta + \cos^2 \theta &= 1
      \end{aligned}$
\end{enumerate}
%
\begin{align}
        \sin^2 \theta + \cos^2 \theta &= 1 \\
        F &= ma \\
        E &= mc^2 \\
        \sin^2 \theta + \cos^2 \theta &= 1
\end{align}
\end{document}

---

Code: `Custom "enumerate":

\documentclass[12pt]{article}
%\usepackage[top=0.3in, bottom=1.2in, left=0.8in, right=0.8in]{geometry}

%\setlength{\parindent}{0cm}

\usepackage[fleqn]{amsmath}

%\usepackage{unicode-math}
%\everymath{\displaystyle}
%\newcommand{\3}{\vspace{0.3cm}}

\newcounter{MyCounter}
\newcommand{\Item}{\refstepcounter{MyCounter}\theMyCounter\quad}
\renewcommand{\theMyCounter}{\arabic{MyCounter})}
\newcommand{\ResetMyCounter}{\setcounter{MyCounter}{0}}

\begin{document}

\begin{gather*}
    \begin{aligned}
        &x+y=0\\
        \Item &y+z=0\\
        \Item &z+x=0
    \end{aligned}
\end{gather*}
And in a subsequent set of equations  \emph{with} \verb|\ResetMyCounter|:
\ResetMyCounter% Comment this out if you wish the numbering to continue
\begin{gather*}
    \begin{aligned}
        &x+y=0\\
        \Item &y+z=0\\
        \Item &z+x=0
    \end{aligned}
\end{gather*}
And in a subsequent set of equations \emph{without} \verb|\ResetMyCounter|:
%\ResetMyCounter% Comment this out if you wish the numbering to continue
\begin{gather*}
    \begin{aligned}
        &x+y=0\\
        \Item &y+z=0\\
        \Item &z+x=0
    \end{aligned}
\end{gather*}
With a \verb|\renewcommand| to use non-numerical numbering:
\renewcommand{\theMyCounter}{\alph{MyCounter}.\quad}
\ResetMyCounter% Comment this out if you wish the numbering to continue
\begin{gather*}
    \begin{aligned}
        &x+y=0\\
        \Item &y+z=0\\
        \Item &z+x=0
    \end{aligned}
\end{gather*}
\end{document}

Answer 2

If I understand your comments to Peter Grill's answer, you want to label some (not all) of your equations and label them on the left. For the small example you give, I think this does roughly what you want, by using leqno in the documentclass and using align rather than aligned inside gather and setting \notag for lines you don't want to have a number.

However, there is probably some good reason why you can't use align or something, that is not clear from your question.

Also, I'm not sure how to get rid of the parentheses around the equation number label if you really want it to look like enumerate.

\documentclass[12pt,leqno]{article}

\usepackage[fleqn]{amsmath}


\begin{document}


\begin{align}
&x+y=0 \\
\intertext{no eq number for the next one} \notag
&y+z=0 \notag \\
&z+x=0 
\end{align}

\end{document}

Answer 3

A solution using the newtagform from \mathtools. With the fleqn option, you have to re-define the values of \mathindent to have the number on the same line as the equation:

\documentclass[12pt,leqno]{article}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage[showframe, nomarginpar]{geometry}
\usepackage[fleqn]{mathtools}%
\newtagform{enum}{\hskip\parindent}{. }
\setlength{\mathindent}{\dimexpr\leftmargini + \parindent\relax}

\begin{document}

\usetagform{enum}
This is an example.
\begin{align}
&x+y=0 \\
\intertext{no eq number for the next one:}  \notag
&y+z=0 \notag \\
&z+x=0
\end{align}

\end{document}