equations
I am trying to write an equation in two lines sharing a single equation number, as shown in the figure.
I tried this code, but it doesn't work in two columns format:
\begin{equation}
\begin{split}
AdditionandMultiplication(a, b_i) & = \sin(p \omega \omega_{4_i}) \\
& AdditionandMultiplication(a, {b}_i + 1) = \tanh(p \omega \omega_{25_i})
\end{split}
\end{equation}
Use the alignedat environment:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{alignedat}{2}
\textup{[M/H]} &\cong 0.917 \textup{[Fe/H]} &&(1)\\
&\cong 0.977 \textup{[Fe/H]} &&(2)\\
&\cong \textup{[Fe/H]} + \log(0.638f_{\alpha}+0.362) \quad&&(3)
\end{alignedat}
\end{equation}
\end{document}
The \; commands you're using are wrong, as they introduce an asymmetric space. Also the chemical symbols should be upright.
I suggest you use a split environment, which in contrast to multline may be used a subenvironment of equation. You need to specify an alignment point on each line with & and separate lines with \\. In this case the first line should be move left relative to the others and the package mathtools provides a convenient command for this:
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\label{wave kinematic}
\begin{split}
\MoveEqLeft
\frac{\partial N(\sigma;\lambda;\theta;t)}{\partial t}
+ \frac{\partial C_{g,\lambda}N(\sigma;\lambda;\theta;t)}
{\partial \lambda} \\
&+ \cos\phi^{-1} \cdot
\frac{\partial C_{f,\phi}N(\sigma;\lambda;\theta;t)}{\partial
\phi} \\
&+ \frac{\partial C_{f,\theta}N(\sigma;\lambda;\theta;t)}{\partial
\theta}
+ \frac{\partial C_{f,\sigma}N(\sigma;\lambda;\theta;t)}{\partial
\sigma}
= \frac{S(\sigma;\theta;\lambda;\varphi;t)}{\sigma}
\end{split}
\end{equation}
\end{document}
I have split across three lines for clarity. If you wanted to just split in two parts, then multlined (notice the extra d) from the mathtools package would be a simpler solution:
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\label{wave kinematic}
\begin{multlined}
\frac{\partial N(\sigma;\lambda;\theta;t)}{\partial t}
+ \frac{\partial C_{g,\lambda}N(\sigma;\lambda;\theta;t)}
{\partial \lambda}
+ \cos\phi^{-1} \cdot
\frac{\partial C_{f,\phi}N(\sigma;\lambda;\theta;t)}{\partial
\phi} \\
+ \frac{\partial C_{f,\theta}N(\sigma;\lambda;\theta;t)}{\partial
\theta}
+ \frac{\partial C_{f,\sigma}N(\sigma;\lambda;\theta;t)}{\partial
\sigma}
= \frac{S(\sigma;\theta;\lambda;\varphi;t)}{\sigma}
\end{multlined}
\end{equation}
\end{document}
All the above works with elsarticle class in your updated question. E.g. the first version becomes:
\documentclass[authoryear,preprint,review,12pt]{elsarticle}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\label{wave kinematic}
\begin{split}
\MoveEqLeft
\frac{\partial N(\sigma;\lambda;\theta;t)}{\partial t}
+ \frac{\partial C_{g,\lambda}N(\sigma;\lambda;\theta;t)}
{\partial \lambda} \\
&+ \cos\phi^{-1} \cdot
\frac{\partial C_{f,\phi}N(\sigma;\lambda;\theta;t)}{\partial
\phi} \\
&+ \frac{\partial C_{f,\theta}N(\sigma;\lambda;\theta;t)}{\partial
\theta}
+ \frac{\partial C_{f,\sigma}N(\sigma;\lambda;\theta;t)}{\partial
\sigma}
= \frac{S(\sigma;\theta;\lambda;\varphi;t)}{\sigma}
\end{split}
\end{equation}
\end{document}
Best Answer
There are (at least) two ways to solve your formatting issue:
alignment only on the
=symbolsalignment both along the left-hand edge and on the
=symbols.Both ways are addressed in the following example. The first way employs an
alignedenvironment, and the second employs analignedat[2]environment. Both environments are explained in depth in the user guide of the amsmath package.