General ⇒ how to make an \underrightarrow in \caption

[lobsterzzz]

Ah, I hope anybody can help me.
I have the following code to input a picture

\begin{figure}
\begin{center}
\includegraphics[width=8cm]{pics/notinward2}
\caption{blablabla}

which works fine.
then I have the following code to make an arrow below the letter x

$\underrightarrow{x}$

which works fine as well. Now I want to have this x with the arrow below in the \caption of the figure.
so I write

\begin{figure}
\begin{center}
\includegraphics[width=8cm]{pics/notinward2}
\caption{blablabla $\underrightarrow{x}$}

but this gives me 4 error messages, which say

illegal parameter number in definition of reserved\@
<to be read again>

What am I doing wrong? how can I do what I want?

[Juanjo]

Instead of $\underrightarrow{x}$, write $\protect\underrightarrow{x}$. This is a subtlety of LaTeX. Commands can be robust or fragile. The latter ones can be prematurely interpreted when used as arguments of other commands, causing compilation errors. This mainly happens when a fragile command appears in a moving argument, like those of \caption, \chapter, \section,... So, fragile commands in such arguments must be "protected" (made robust), by prefixing them with the \protect command.

Tipically, commands with optional arguments are fragile. This is why the following code fails:

\section{\framebox[12em]{Section title}}

However, we can protect \framebox and solve the problem:

\section{\protect\framebox[12em]{Section title}}

So, coming back to your problem, it seems that \underrightarrow is fragile. It suffices to protect it.