[Tex/LaTex] Numbering an equation when using \displaymath

labelsmath-mode

I am using the hyperref package and want to label a piecewise equation. However, I only know how to use \displaymath to do a piecewise equation and the \label doesn't put a number next to the equation. Any suggestions?

\documentclass[12pt]{amsart}

\usepackage{amsfonts,amssymb,amsmath,graphicx,hyperref}



\begin{document}


\begin{displaymath} \label{eq:torus}
   A_{T(p,q)}(z,w)  = \left\{
     \begin{array}{ll}
       w(w+2qz - \frac{1}{2}) & : p =2, q>2 \\
\\
       w(w + pqz - \frac{1}{2})(w+pqz) & : p,q > 2 
     \end{array}
   \right.
\end{displaymath}

\end{document}

Best Answer

The displaymath environment does not provide an equation number:

\begin{displaymath} \label{somelabel}

will use the last counter being used in the \refstepcounter call and this is certainly not what is wanted here.

To get the labels it is necessary to use a numbering math environment, e.g. equation or align and then use the \begin{cases}...\end{cases} environment for the specific setup of a piecewise function.

\documentclass[12pt]{amsart}

\usepackage{amsfonts,amssymb,amsmath,graphicx,hyperref}

\begin{document}


%\begin{displaymath}  \label{eq:torus}
 %
%A_{T(p,q)}(z,w) = \left\{
%     \begin{array}{ll}
%       w(w+2qz - \frac{1}{2}) & : p =2, q>2 \\
%\\
%       w(w + pqz - \frac{1}{2})(w+pqz) & : p,q > 2 
%     \end{array}
%   \right.
%
%\end{displaymath}


\begin{equation}
  A_{T(p,q)}(z,w) = \begin{cases}
    w(w+2qz - \frac{1}{2}) & : p =2, q>2 \\
    w(w + pqz - \frac{1}{2})(w+pqz) & : p,q > 2  
  \end{cases} \label{eq:torus}
\end{equation}

\end{document}

enter image description here

Related Solutions

[Tex/LaTex] Why does \widehat behave differently if I insert \hspace{0pt}

If you put an accent over a single character, TeX uses information in the font metrics to shift the accent to take some account of the slope of the italic letters. Which is why the first one shifts. If you put an accent over a more complicated math list then it's just centered over the list.

[Tex/LaTex] Numbering equation using eqnarray*

In addition to recommending that you use the align environment (provided by the amsmath package) instead of the eqnarray* environment, I would also suggest you make each line's first + symbol the alignment point, rather than the = symbol; that way, the structure of the equation should become very readily apparent. And, since the string \bigg|_{z = z_{0}} occurs no fewer than six times, it's handy to create a shortcut macro for it.

enter image description here

\documentclass{article}
\usepackage{amsmath} % provides the "align" environment
\newcommand\atz{\bigg\vert_{z=z_0}\!} % a shortcut macro
\begin{document}
\begin{align} 
f_{\textit{lin}}(z) =  f(z_{0}) 
&+ 
\frac{\partial f(z)}{\partial \ddot{\theta}}\atz \ddot{\theta} + 
\frac{\partial f(z)}{\partial \ddot{\alpha}}\atz \ddot{\alpha} + 
\frac{\partial f(z)}{\partial \dot{\theta}}\atz \dot{\theta} \notag \\ 
&+ \frac{\partial f(z)}{\partial \dot{\alpha}}\atz \dot{\alpha} + 
\frac{\partial f(z)}{\partial \theta}\atz \theta + 
\frac{\partial f(z)}{\partial \alpha}\atz \alpha
\end{align}
\end{document}

If you don't want to use the align environment, you could use a split environment inside an equation environment; the main difference is that there's a single equation number that's centered vertically across the rows of the equation:

enter image description here

\documentclass{article}
\usepackage{amsmath}
\newcommand\atz{\bigg\vert_{z=z_0}\!} % a shortcut macro
\begin{document}
\begin{equation}
\begin{split} 
f_{\textit{lin}}(z) =  f(z_{0}) 
&+ 
\frac{\partial f(z)}{\partial \ddot{\theta}}\atz \ddot{\theta} + 
\frac{\partial f(z)}{\partial \ddot{\alpha}}\atz \ddot{\alpha} + 
\frac{\partial f(z)}{\partial \dot{\theta}}\atz \dot{\theta} \\[1ex] % some additional vertical separation
&+ \frac{\partial f(z)}{\partial \dot{\alpha}}\atz \dot{\alpha} + 
\frac{\partial f(z)}{\partial \theta}\atz \theta + 
\frac{\partial f(z)}{\partial \alpha}\atz \alpha
\end{split}
\end{equation}
\end{document}

Best Answer

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[Tex/LaTex] Numbering equation using eqnarray*

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