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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
  • 1
    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
3

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.
<to be read again> 
                   _
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

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