2

In this example the equation number is even below the equation itself.
I thought a [t] of [m/c] option could position the number. Doesn't work.

How can I get the equation number to be at the top, centered, at the bottom?

\documentclass{article}

\usepackage{showframe}   %% just for demo
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{amsthm}
\usepackage{mathtools}
\usepackage{relsize}
\usepackage{stmaryrd}
\usepackage{bm}

\newcommand{\h}[1]{\widehat{\bm{#1}}}

\begin{document}

\begin{align}
\begin{aligned}[t]
\hspace{3cm} R(\alpha \h{a})    &\hspace{1cm}  \longmapsto  \hspace{1cm}  a.
\intertext{Note the special cases for the identity and the $\pi$-rotation:}
\mbox{Identity}    &\hspace{1cm}   \longmapsto     \hspace{1cm}  1         \equiv  -1 \\
\mbox{$L_a$}       &\hspace{1cm}    \longmapsto    \hspace{1cm}  a   \equiv  - a.
\end{aligned}
\end{align}

\begin{equation}
\begin{split}
\hspace{3cm} R(\alpha \h{a})    &\hspace{1cm}  \longmapsto  \hspace{1cm}  a.
\intertext{Note the special cases for the identity and the $\pi$-rotation:}
\mbox{Identity}    &\hspace{1cm}   \longmapsto     \hspace{1cm}  1         \equiv  -1 \\
\mbox{$L_a$}       &\hspace{1cm}    \longmapsto    \hspace{1cm}  a   \equiv  - a.
\end{split}
\end{equation}

\end{document}
2

Here is a solution where everything is aligned, and one equation number for the whole set placed with any of the equation lines, or even with the intertext (emulated in the latter case with the flalign environment and the \rlap command):

\documentclass{article}

\usepackage[nomarginpar, showframe]{geometry} %% just for demo
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{amsthm}
\usepackage{stmaryrd}
\usepackage{bm}

\newcommand{\h}[1]{\widehat{\bm{#1}}}

\begin{document}

\begin{align}
  \hspace{3cm} R(\alpha \h{a}) &\hspace{1cm} \longmapsto \hspace{1cm} a.\\
\intertext{\rlap{Note the special cases for the identity and the $\pi$-rotation:}}
\mbox{Identity} &\hspace{1cm} \longmapsto \hspace{1cm} 1 \equiv -1\notag\\
  L_a &\hspace{1cm} \longmapsto \hspace{1cm} a \equiv - a. \notag
\end{align}


\begin{align}
  \hspace{3cm} R(\alpha \h{a}) &\hspace{1cm} \longmapsto \hspace{1cm} a. \notag\\
\intertext{\rlap{Note the special cases for the identity and the $\pi$-rotation:}}
\mbox{Identity} &\hspace{1cm} \longmapsto \hspace{1cm} 1 \equiv -1\\
  L_a &\hspace{1cm} \longmapsto \hspace{1cm} a \equiv - a. \notag
\end{align}

 \begin{align}
  \hspace{3cm} R(\alpha \h{a}) &\hspace{1cm} \longmapsto \hspace{1cm} a. \notag \\
\intertext{\rlap{Note the special cases for the identity and the $\pi$-rotation:}}
\mbox{Identity} &\hspace{1cm} \longmapsto \hspace{1cm} 1 \equiv -1 \notag \\
  L_a &\hspace{1cm} \longmapsto \hspace{1cm} a \equiv - a.
\end{align}

\begin{flalign}
  & & \hspace{3cm} R(\alpha \h{a}) &\hspace{1cm} \longmapsto \hspace{1cm} a. & & \notag\\[8pt]
 & \text{\rlap{Note the special cases for the identity and the $\pi$-rotation:}} \\[8pt]
 & & \mbox{Identity} &\hspace{1cm} \longmapsto \hspace{1cm} 1 \equiv -1 \notag\\
  & & L_a &\hspace{1cm} \longmapsto \hspace{1cm} a \equiv - a. \notag
\end{flalign}

\end{document} 

enter image description here

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.