# A lot of undefined control sequences and missing { } $errors I don't want to open up too many threads so I'm combining two errors together. Sorry in advance. The full paragraph in question \documentclass {book} \include {amsmath} \begin {document} \section {error prone} The charge density is substituted by a superposition of a reference or input density$ n'_{0} = n_{0} ( \vec{r}' ) $and a small change of$ \delta n' \eq \delta n (\vec {r}') , \int d \vec_{r}' $is expressed by$\int '$\end {document}  These lines are having undefined control sequences This other error is  ! Missing { inserted. <to be read again> ? ! Missing } inserted. <inserted text> ? ! Missing { inserted. <to be read again> ? ! Missing } inserted. <inserted text> ? ! Missing$ inserted.
<inserted text>


I have included {amsmath}

• please always post a complete test document not a fragment. \eq is not defined by default, just use = – David Carlisle Mar 14 '18 at 11:25
• @DavidCarlisle done. – user121392 Mar 14 '18 at 11:32
• no a test document is a complete document that peopel can run to produce the error. – David Carlisle Mar 14 '18 at 11:38
• \vec{r} would not generate an error but is that the output you want? Did you intende to make something a subscript with \vec_{r} ? – David Carlisle Mar 14 '18 at 11:39
• @DavidCarlisle Im not sure. The dvi output displays it just fine, but it does throw the error. Yes I am looking to vectorize r – user121392 Mar 14 '18 at 11:44

You neither showed the full document nor the full error message.

\documentclass{article}

\begin{document}
The charge density is substituted by a superposition of a reference or
input density $n'_{0} = n_{0} ( \vec{r}' )$and a small change of
$\delta n' \eq \delta n (\vec {r}') , \int d \vec_{r}'$ is expressed by $\int '$
\end{document}


Produces the error

! Undefined control sequence.
l.6 $\delta n' \eq \delta n (\vec {r}') , \int d \vec_{r}'$ is expressed b...

?


Where the linebreak shows how far TeX had got, and that the undefined command is \eq. that should be =.

The document

\documentclass{article}

\begin{document}
The charge density is substituted by a superposition of a reference or
input density $n'_{0} = n_{0} ( \vec{r}' )$and a small change of
$\delta n' = \delta n (\vec {r}') , \int d \vec_{r}'$ is expressed by $\int '$
\end{document}


produces the error

! Missing { inserted.
_
l.6 ...ta n' = \delta n (\vec {r}') , \int d \vec_
{r}' $is expressed by$\i...

?


where again the linebreak highlights where the error is.

\documentclass{article}

\begin{document}
The charge density is substituted by a superposition of a reference or
input density $n'_{0} = n_{0} ( \vec{r}' )$and a small change of
$\delta n' = \delta n (\vec {r}') , \int d \vec{r}'$ is expressed by $\int '$
\end{document}


runs without error.

• Thank you. I was wondering why the line breaks appear during the output. – user121392 Mar 14 '18 at 11:47