caption in a table containing matrix

Question

I put a matrix expression in a table. Now I'm trying to insert a caption, but I obtain an error and caption don't appear in my pdf. This is my code:

\begin{table*}
\[
\begin{bmatrix}
y_\triangle \\
y
\end{bmatrix}
\Large
=
\renewcommand\arraystretch{2}
\begin{bmatrix}[c|ccc]
- \frac{\omega_i C C_{dyn} H G}{1+C C_{dyn} H G} & \frac{\omega_i C C_{dyn}}{1+C C_{dyn} H G} & \frac{\omega_i}{1+C C_{dyn} H G} & - \frac{\omega_i C C_{dyn} H}{1+C C_{dyn} H G} \\ \hline
\frac{\omega_p G}{1+C C_{dyn} H G} & \frac{\omega_p C C_{dyn} G}{1+C C_{dyn} H G} & \frac{\omega_p G}{1+C C_{dyn} H G} & - \frac{\omega_p C C_{dyn} H G}{1+C C_{dyn} H G}
\end{bmatrix}
\normalsize
\renewcommand\arraystretch{1}
\begin{bmatrix}
u_\triangle \\
r\\
\omega \\
n
\end{bmatrix}
\]
\caption{Trasformata U_LFT del sistema}
\label{fig:ulft}
\end{table*}

In the preamble I inserted this code for insert vertical line in the matrix.

\makeatletter
\renewcommand*\env@matrix[1][*\c@MaxMatrixCols c]{%
  \hskip -\arraycolsep
  \let\@ifnextchar\new@ifnextchar
  \array{#1}}
\makeatother

Thanks to everybody for help

Answer 1

To get rid of the error message generated by the caption, change _ to \_ in the argument of \caption.

You also should contemplate fixing several other issues. Do get rid of the font size changes within the formula, the \hline directive, and the changes to \arraystretch. Use \dfrac instead of \frac if you want larger fraction terms.

\documentclass{article}
\usepackage[italian]{babel}
\usepackage{amsmath} % for 'bmatrix' environment and '\dfrac' macro
\usepackage[a4paper,margin=2.5cm]{geometry} % set page and text block parameters
\begin{document}
\begin{table}
\[
\begin{bmatrix} y_\triangle \\ y \end{bmatrix}
%%%\Large
=
%%%\renewcommand\arraystretch{2}
\begin{bmatrix}%%%[c|ccc]
-\dfrac{\omega_i C C_{\mathrm{dyn}} H G}{1+C C_{\mathrm{dyn}} H G} & 
 \dfrac{\omega_i C C_{\mathrm{dyn}}}{1+C C_{\mathrm{dyn}} H G} & 
 \dfrac{\omega_i}{1+C C_{\mathrm{dyn}} H G} & 
-\dfrac{\omega_i C C_{\mathrm{dyn}} H}{1+C C_{\mathrm{dyn}} H G} \\[3ex] %%%\hline
 \dfrac{\omega_p G}{1+C C_{\mathrm{dyn}} H G} & 
 \dfrac{\omega_p C C_{\mathrm{dyn}} G}{1+C C_{\mathrm{dyn}} H G} & 
 \dfrac{\omega_p G}{1+C C_{\mathrm{dyn}} H G} & 
-\dfrac{\omega_p C C_{\mathrm{dyn}} H G}{1+C C_{\mathrm{dyn}} H G}
\end{bmatrix}
%%%\normalsize
%%%\renewcommand\arraystretch{1}
\begin{bmatrix} u_\triangle \\ r\\ \omega \\ n \end{bmatrix}
\]
\caption{Trasformata U\_LFT del sistema} % <--- change "_" to "\_"
\label{fig:ulft}
\end{table}
\end{document}

---

Addendum: If you must insert a vertical and a horizontal line in the 2x4 matrix, I suggest you do so by using an array environment, rather than a bmatrix environment. That way, you won't have \hline generating a line that's too long and ends up interfering with the square brackets.

\left[ \begin{array}{@{}c|ccc@{}}
-\dfrac{\omega_i C C_{\mathrm{dyn}} H G}{1+C C_{\mathrm{dyn}} H G} & 
\dfrac{\omega_i C C_{\mathrm{dyn}}}{1+C C_{\mathrm{dyn}} H G} & 
\dfrac{\omega_i}{1+C C_{\mathrm{dyn}} H G} & 
- \dfrac{\omega_i C C_{\mathrm{dyn}} H}{1+C C_{\mathrm{dyn}} H G} \\[2ex]
\hline 
% use '\mathstrut' to get a bit more vertical separation:
\dfrac{\omega_p G^{\mathstrut}}{1+C C_{\mathrm{dyn}} H G} & 
\dfrac{\omega_p C C_{\mathrm{dyn}} G}{1+C C_{\mathrm{dyn}} H G} & 
\dfrac{\omega_p G}{1+C C_{\mathrm{dyn}} H G} & 
- \dfrac{\omega_p C C_{\mathrm{dyn}} H G}{1+C C_{\mathrm{dyn}} H G}
\end{array}\right]