Writing a formula below a matrix

Question

I have a matrix environment:

\begin{bmatrix}

 ...

\end{bmatrix}

How can I write below that matrix? I tried with substack, so I wrote \end{bmatrix}_{\substack{...}}

That doesn't work as it does with, for example, a sum. How can I write a formula exactly below the matrix?

EDIT: Okay, I found out how to do it myself now by using the array environment

\documentclass[a4paper,12pt, oneside]{article}
\usepackage{amsmath,amsfonts,amssymb,amsopn,amscd}

\begin{document}

\begin{equation*}
\begin{array}{c}
A : =
\begin{bmatrix}
x \\
y \\
z \\
a
\end{bmatrix} \\
x=0
\end{array}
\end{equation*}

\end{document}

Answer 1

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}

Answer 2

An equation below the matrix in normal size looks a little odd, if the equation is just an annotation/index to the matrix. The following uses the same method as putting equations below \sum, \prod and other operators:

\documentclass[a4paper,12pt, oneside]{article}
\usepackage{amsmath,amsfonts,amssymb,amsopn,amscd}
\usepackage{colonequals}

\begin{document}

\begin{equation*}
A \colonequals
\mathop{
  \begin{bmatrix}
  x \\
  y \\
  z \\
  a
  \end{bmatrix}
}\limits_{x=0}
\end{equation*}

\end{document}

Answer 3

We can use \underset

\documentclass[a4paper,12pt, oneside]{article}

\usepackage{amsmath,amsfonts,amssymb,amsopn,amscd}

\begin{document}

$A:=\underset{
\begin{array}{c}
x=0
\end{array}
}{
\begin{bmatrix}
x \\ 
y \\ 
z \\ 
a
\end{bmatrix}
}$



\end{document}

to create

ADDED. Comparison between the above code and as commented by egreg:

\documentclass[a4paper,12pt, oneside]{article}
\usepackage{amsmath,amsfonts,amssymb,amsopn,amscd}
\begin{document}
Original

$A:=\underset{
\begin{array}{c}
x=0
\end{array}
}{
\begin{bmatrix}
x \\ 
y \\ 
z \\ 
a
\end{bmatrix}
}$

As per egreg's comment

$A:=\underset{
\begin{array}{@{}c@{}}
x=0
\end{array}
}{
\begin{bmatrix}
x \\ 
y \\ 
z \\ 
a
\end{bmatrix}
}$
\end{document}

Output