matrices
My Jacobian matrix evaluated at the equilibrium is denoted by J_E. I typed for its trace as
tr\textit{J_E}.
And the result is very strange. tr appears good but \textit{J_E} part is unlimitedly repeated with its size gradually decreasing.
It looks something like:
where the J_E part is gradually getting smaller.
What is happening here?
With amsmath you can use
\xrightarrow{r_{2}-r_{1}}
that will produce an arrow wide enough to accommodate the thing on top.
In order to split matrices you need to use array
\left[\begin{array}{@{}cc|c@{}}
<entries>
\end{array}\right]
With the @{} at both sides you'll get output similar to that of bmatrix.
Since you are already using amsmath, it's better to use its environments:
\begin{equation*}
\begin{matrix}
A : =
\begin{bmatrix}
x \\
y \\
z \\
a
\end{bmatrix} \\
x=0
\end{matrix}
\end{equation*}
In this case the difference is not evident, but array adds spaces at both sides that matrix doesn't.
If you want that the condition is exactly below the matrix and not centered also with respect to A:=, then an array is the way to go, but in a more sophisticated fashion:
\documentclass{article}
\usepackage{mathtools} % load also amsmath
\begin{document}
\begin{equation*}
\begin{array}{@{}c@{}c@{}}
A : = {} &
\begin{bmatrix}
x \\
y \\
z \\
a
\end{bmatrix} \\
& \mathclap{x=0}
\end{array}
\end{equation*}
\end{document}
Best Answer
Why should you type
\textit{J_E}? This is wrong for several reasons._requires math mode and the argument to\textitis typeset in text mode (as the command name suggests).\textitis the wrong command for typesetting a math variable: math mode should be used.You're right that “tr” for the trace should be in upright type, but typesetting a formula not in math mode just for this is misusing the tool.
Now, why the four repetitions? You first of all get an error message that says
(the line number can be different, of course). After this the output is unpredictable, because when TeX discovers the error it's too late for doing a sensible fix.