2

I wonder how environment \subequations is different from \gather. I know the numbering format for the two is a little bit different and you can refer to a single equation in \subequations environment. But apart from that, are there any other differences? And when should one use \subequations as well as \gather?

Attached is a MwE.

\documentclass[a4paper]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsfonts}
\begin{document}
\begin{subequations}\label{eq:joint}
  \begin{equation}\label{eq:first}
    A^{[2]} \diamond B^{[2]} \cong (A \diamond B)^{[2]}
  \end{equation}
  \begin{equation}\label{eq:second}
    A^{\langle 2 \rangle} \diamond B^{\langle 2 \rangle} \cong (A \diamond B)^{\langle 2 \rangle}
  \end{equation}
\end{subequations}

\begin{gather}
  A^{[2]} \diamond B^{[2]} \cong (A \diamond B)^{[2]} \\
  A^{\langle 2 \rangle} \diamond B^{\langle 2 \rangle} \cong (A \diamond B)^{\langle 2 \rangle}
\end{gather}

\end{document} 
3

They have rather different use cases. subequations only affects the numbering, and is used when you have a series of equations that should share a common number. However, it is not a math environment by itself, i.e. it doesn't enter math mode. It only affects the numbering of numbered equations that are within it.

gather is used when you have a several displayed equations that should be typeset right after one another. You shouldn't use multiple equation environments in a row, that is precisely the case where gather (or align) should be used.

If you have two equations that should be typeset as in your example, one right after the other, and you want the numbering of those to be of the form 1a, 1b, then you would want to have both subequations and gather.

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