# Equation numbering not shown

I am writing an equation using the syntax s follows:-

{
$N\!D_{\max} = \begin{cases} \frac{n}{2} &: \text{n even,} \\ \frac{n-1}{2} &: \text{n odd.} \end{cases}$  }\label{form:19}


I am able to label the equation during a sentence but the same equation number is not being shown beside the equation. How can I fix this problem? A picture is attached for better understanding.

• As always on the this site: please post a full minimal example instead of sniplet. $...$  dies not number equations so I don't know what you are expecting with that label. – daleif Oct 27 '18 at 10:48

You can use equation environment to get equation numbers or you can use \tag statement to give whatever equation number that you want to give. {
$N\!D_{\max} = \begin{cases} \frac{n}{2} &: \text{n even,} \\ \frac{n-1}{2} &: \text{n odd.} \end{cases}$  }\label{form:19}

Referring to Equation~\ref{form:19}.

{
$N\!D_{\max} = \begin{cases} \frac{n}{2} &: \text{n even,} \\ \frac{n-1}{2} &: \text{n odd.} \end{cases}\tag{3.2}\label{form:19a}$  }

Referring to Equation~\ref{form:19a}.

\begin{equation}
N\!D_{\max} =
\begin{cases}
\frac{n}{2} &:  \text{$n$   even,} \\
\frac{n-1}{2} &: \text{$n$  odd.}
\end{cases}
\label{form:19b}
\end{equation}

Referring to Equation~\ref{form:19b}.


The braces around the equation are wrong; also, if you want a numbered equation you need equation, not $...$. The \label should go inside it.

Here's a corrected version, using some more facilities from mathtools, for instance dcases* (see its manual).

\documentclass{article}
\usepackage{amsmath,mathtools}

\newcommand{\ND}{\mathit{ND}}

\begin{document}

The nodal diameter ($\ND$) of a deformation or excitation shape
is equal to the number of inflection lines'' across the disk.
The maximum nodal diameter $\ND_{\max}$ is given by Eq.~\eqref{form:19}
where $n$ denotes the number of periodic sections
\begin{equation}\label{form:19}
\ND_{\max}=
\begin{dcases*}
\frac{n}{2}   & $n$ even, \\
\frac{n-1}{2} & $n$ odd.
\end{dcases*}
\end{equation}

\end{document} 