\label inside an align environment [closed]

Question

It seems like whenever I use \label{an-label} inside the align environment, and then use to refer to it using \refeq{an-label}, I get the section number instead of the equation number. This does not happen when I use the equation environment.

For example:

\begin{align}
f(x) & = x^2 \label{label1}
\end{align}

Equation \refeq{label1}

would display the section number instead of the equation number. I use mathtools. Is there any reason for this to happen?

When looking in google, it is stated in some web page that \begin{align} \end{align} should be used with the \label{} to name an equation, so I am not sure what I am doing wrong.

Answer 1

I don't know why you're using the align environment for a single equation instead of the equation environment. Anyway, I've tried this code and it works fine:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\section{Section 1}
\label{sec1}
 This is section \ref{sec1}.
 \section{Section 2}
 \label{sec2}
 This is section \ref{sec2}
 \begin{equation}
 2+2=4
 \end{equation}
\begin{equation}
 3+3=6
 \end{equation}
 \begin{align}
 f(x)&=x^2 \label{ali1}
 \end{align}
 Take a look at equation \eqref{ali1}.
\end{document}

I've tried using the "mathtools" package instead of "amsmath" but I didn't found any problem... I don't realize what is really happening...

Answer 2

I had the same issues like Stefan, however with "gather". Since I found the answer to his issue and also could reproduce the "wrong" referencing, I would like to share my findings with you.

It depends where you put the \label in your equation environment. Working example: In this example, you get the correct label for the one label that you assign to the equations.

\begin{gather}
\arraycolsep=3pt
\medmuskip = 1mu
{^{O}{\dot{\psi}(t)}} = \int^{t}_{0} \frac{{^{O}\V{\tau}_{G,z}}}{{^{B}{J}_{zz}}} dt + {^{O}{\dot{\psi}(t=0)}}, \nonumber \\
{^{O}{\psi(t)}} = \int^{t}_{0} (\int^{t}_{0} \frac{{^{O}\V{\tau}_{G,z}}}{{^{B}{J}_{zz}}} dt + {^{O}{\dot{\psi}(t=0)}}) dt \nonumber \\
\label{calcAngularPosVel} + {^{O}{\psi}(t=0)}, \\
{^{O}{\dot{\psi}(t)}} = {^{B}{\dot{\psi}(t)}}, {^{O}{\psi(t)}} = {^{B}{\psi(t)}}. \nonumber 
\end{gather}
\ref{calcAngularPosVel}

Example with "wrong" reference: In this example, you would get the labeling of the section, not for the equation, since the label tries to reference the last line of all equations provided in the gather environment. Hovewer, the last equation line does not provide a label, since the \nonumber command suppresses this. Thus, latex cannot reference to this equation.

\begin{gather}
\arraycolsep=3pt
\medmuskip = 1mu
{^{O}{\dot{\psi}(t)}} = \int^{t}_{0} \frac{{^{O}\V{\tau}_{G,z}}}{{^{B}{J}_{zz}}} dt + {^{O}{\dot{\psi}(t=0)}}, \nonumber \\
{^{O}{\psi(t)}} = \int^{t}_{0} (\int^{t}_{0} \frac{{^{O}\V{\tau}_{G,z}}}{{^{B}{J}_{zz}}} dt + {^{O}{\dot{\psi}(t=0)}}) dt \nonumber \\
+ {^{O}{\psi}(t=0)}, \\
{^{O}{\dot{\psi}(t)}} = {^{B}{\dot{\psi}(t)}}, {^{O}{\psi(t)}} = {^{B}{\psi(t)}}. \nonumber
\label{calcAngularPosVel}  
\end{gather}
\ref{calcAngularPosVel}

So it depends where you put the \label command if you want to reference the entire system of equations with only one reference.

Hope this is of help for anyone.