equations
How to set the numbering of an equation set to align bottom+right?
For example, in following code. The default numbering will be placed on the same row as S. But is it possible to change it to row 3 (the P = ??? row)?
\begin{equation}
\begin{split}
A &= 0011 0000 0_2\\
S &= 1101 0000 0_2\\
P &= 0000 1100 0_2
\end{split}
\end{equation}
My answer https://tex.stackexchange.com/a/136374/15925 may be adapted to introduce a command \cmdcol for material to be centered in a column in display math in alignat and related environments:
\documentclass{book}
\usepackage{mathtools}
\numberwithin{equation}{section}
\makeatletter
\newcommand{\cmdcol}[1]{\omit\hfil\strut@ \( \m@th\displaystyle #1 \)\hfil}
\makeatother
\begin{document}
\chapter{}
\section{Section}
\subsection{Subsection}
\begin{equation}
1 + 1 = 2
\label{eq:1}
\end{equation}
\begin{alignat}{2}
E &= n^\mu n^\nu T_{\mu\nu} &\cmdcol{=}& \rho \notag \\
T &= T^\mu_\mu = -\rho + p_r + 2 p_\perp
&\cmdcol{\overset{\eqref{eq:1}}{=}}& 2 (p_\perp -\rho) \notag \\
S &= T+E &\cmdcol{=}& 2 p_\perp - \rho \notag \\
p^\mu &= - \gamma^{\mu \alpha} n^\beta T_{\alpha \beta} &\cmdcol{=}& 0 \quad .
\end{alignat}
\end{document}
The structure of the \cmdcol command is built up to mimic the standard ams definition for columns in align and friends.
By the way, your numbering scheme is better effected by \numberwithin. I have switch to mathtools rather than amsmath for the further extensions provided by that package, it loads amsmath anyway.
The gathered environment has been designed for this purpose.
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{gathered}
i^2 = j^2 = k^2 = -1\\
ij = -ji = k
\end{gathered}
\end{equation}
\end{document}
Best Answer
Use the
tbtagsoption to theamsmathpackage:tbstands for "top or bottom"; when the equation numbers are on the right the tag will be placed at the bottom of the block, when the numbering is on the left, the tags will be placed at the top. The opposite option iscentertags. Thesplitenvironment is defined by theamsmathpackage, but if you loadmathtoolsinstead you can write\usepackage[tbtags]{mathtools}which will pass the option on toamsmathwhenmathtoolsloads it.