Invalid use of \intertext

Question

I'm writing a report. I first did it on ShareLaTeX and it compiled successfully. Now, I'm redoing it on texmaker and it doesn't compile stating the following error: "invalid use of \intertext".

The part on the code in which I get the error is this one:

\begin{subequations}
\begin{align}\label{eq:twoa}
    \begin{split}
    J\dot{\omega}=rF_z \mu \left(\dfrac{v-\omega r}{v}\right)-T_b \\
    m\dot{v}=-F_z \mu \left(\dfrac{v-\omega r}{v}\right)
    \end{split}      
\end{align}
\intertext{\noindent In (\ref{eq:twoa}) the state variables are $\omega$ and $v$. Since $\omega$, $v$, and $\lambda$ are linked by an algebraic relationship, it is possible to replace the state variable $\omega$ with the state variable $\lambda$. This can be simply obtained by plugging the following two relationships 
\begin{align*}
    \dot{\lambda}=-\dfrac{r}{v}\dot{\omega} + \dfrac{r\omega}{v^2}\dot{v}, \hspace{4mm} \omega=\dfrac{v}{r}(1-\lambda)
\end{align*}
into the first equation of (\ref{eq:twoa}), so obtaining:}

\begin{align}\label{eq:twob}
    \begin{split}
    \dot{\lambda}=-\dfrac{1}{v}\left(\dfrac{(1-\lambda)}{m}+\dfrac{r^2}{J}\right) F_z \mu(\lambda) + \dfrac{r}{vJ}T_b \\
    m\dot{v}=-F_z\mu(\lambda)
    \end{split}      
\end{align}
\end{subequations}

And actually the output is this one in ShareLaTeX (what I need):

But I can't make it work in texmaker, I keep getting the "invalid use of \intertext" error message. What should I do to fix this on texmaker? Or how could I get the same output properly?

Answer 1

You don't really need \intertext at all here, just remove it. Also, don't use align for single-line equations (align vs equation).

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{subequations}
\begin{equation}\label{eq:twoa}
    \begin{split}
    J\dot{\omega}=rF_z \mu \left(\dfrac{v-\omega r}{v}\right)-T_b \\
    m\dot{v}=-F_z \mu \left(\dfrac{v-\omega r}{v}\right)
    \end{split}      
\end{equation}
In (\ref{eq:twoa}) the state variables are $\omega$ and $v$. Since $\omega$, $v$, and $\lambda$ are linked by an algebraic relationship, it is possible to replace the state variable $\omega$ with the state variable $\lambda$. This can be simply obtained by plugging the following two relationships 
\begin{equation*}
    \dot{\lambda}=-\dfrac{r}{v}\dot{\omega} + \dfrac{r\omega}{v^2}\dot{v}, \hspace{4mm} \omega=\dfrac{v}{r}(1-\lambda)
\end{equation*}
into the first equation of (\ref{eq:twoa}), so obtaining:
\begin{equation}\label{eq:twob}
    \begin{split}
    \dot{\lambda}=-\dfrac{1}{v}\left(\dfrac{(1-\lambda)}{m}+\dfrac{r^2}{J}\right) F_z \mu(\lambda) + \dfrac{r}{vJ}T_b \\
    m\dot{v}=-F_z\mu(\lambda)
    \end{split}      
\end{equation}
\end{subequations}
\end{document}

Regarding ShareLaTeX, I guess you see something like this:

Notice that little red box with the number 1 in it? That means that you got one error. While the output of a document with errors is sometimes correct, that is just luck, so you should always make sure to not have any errors. If you click that icon with the red box, you'll see what the error says: