# [Tex/LaTex] Equation numbering gone wrong

equationsnumberingsubequations

There are three issues that I would like to fix which are probably interlinked, but don't know how (have tried a couple of things):

1) Force the equation numbering of equation 3.7 to move to the right-hand side of the the lower equation (underneath the updownarrow, so it looks like the numbering of equation 3.8.
2) Avoid compressing the size of the lower equation
3) Avoid italic font of equation 3.8

This is my code:

\documentclass[12pt,twoside]{book}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{amsmath}
\begin{document}

$$\frac{c_r(6\sigma+3)+4\sigma^2(\sigma-2P_c+1)-2P_c+1}{(2\sigma+1)^2} = 0 \\ \center\Updownarrow \\ P_c^* = \frac{4\sigma^2+4\sigma+1+4\sigma^3+6c_r\sigma+3c_r}{2(1+4\sigma)}\center \label{3.7}$$
\vspace{-3cm}

The original paper produces the following result:

$$P_c^* = \frac{4\sigma^2+4\sigma+1}{2(1+4\sigma)} \label{3.8}$$

\end{document}


You cannot break lines in equation. Use a gathered environment within equation. The label will be typeset between the equation lines, unless you load amsmath with the tbtags option. If you want the label to appear on the last line, you can use the gather environment, and \notag on the first two lines:

\documentclass{article}
\usepackage{mathtools}
\numberwithin{equation}{section}

\begin{document}
\setcounter{section}{3}
$$\begin{gathered} \frac{c_r(6\sigma+3)+4\sigma²(\sigma-2P_c+1)-2P_c+1}{(2\sigma+1)²} = 0 \\ \Updownarrow \\ P_c^* = \frac{4\sigma²+4\sigma+1+4\sigma³+6c_r\sigma+3c_r}{2(1+4σ)} \end{gathered}$$

\begin{gather}
\notag \frac{c_r(6\sigma+3)+4\sigma²(\sigma-2P_c+1)-2P_c+1}{(2\sigma+1)²} = 0 \\
\notag \Updownarrow \\
P_c^* = \frac{4\sigma²+4\sigma+1+4\sigma³+6c_r\sigma+3c_r}{2(1+4σ)}
\label{3.7bis}
\end{gather}

\end{document}