missing $ inserted

Question

Hi I have trubles with compiling this

    \begin{equation}
J = \sum_{j=1}^{C}
\end{equation}

I constantly gets an error

missing $ inserted

which is quite strange because I'm pretty sure that I'm in math environment I alsow tried this

\begin{equation}
J = $\sum_{j=1}^{C}$
\end{equation}

or this

\begin{equation}
J = $\sum_{$j=1$}^{$C$}$
\end{equation}

but it didnt't do the trick

EDIT The only thing above is

 \begin{equation}
 d_{ij}^2 = || x_{i} - p_{ij}||^2 = || x_{i} - (R_{j}s_{j}p_{ij}^* + t_{j}  ) ||^2

\end{equation}

However if I delete this the document compile. It seems that in this section there is some error

Solution

You have a double newline (i.e. a blank line) in the second code snippet. This causes TeX to drop out of math mode in an attempt to correct this error, hence the subsequent errors about missing $s.

Comments

[Aasmund Eldhuset]

Indeed, your first code snippet seems correct. Are you absolutely sure that there are no mismatched dollar signs elsewhere in the document?

[SiggyF]

This equation seems fine, might it be somewhere else in your document? Maybe try to comment this section out. Do you have any special characters outside of an equation or any loose $?

[Ulrich Schwarz]

Try ignoring the error (by answering Q at the error prompt) and looking at the output. The place where TeX inserted the $ is obvious because all the letters turn to maths.

[nina]

I edited the question. Please take a look

[Aasmund Eldhuset]

Interesting; I didn't know that that could cause problems. +1.