How can I align a text right below a bmatrix / pmatrix?

Question

Might be a dumb question, but I am firing it:

How can I align a text, A, in this case, right under a bmatrix/pmatrix?

I currently have this code:

        \begin{equation*}
        \begin{pmatrix}
            3 & -1 & 1 \\
            3 & 3  & 7 \\
            3 & 6  & 2
        \end{pmatrix}\begin{pmatrix}
            x_1 \\
            x_2 \\
            x_3
        \end{pmatrix} = \begin{pmatrix}
            1 \\
            4 \\
            0
        \end{pmatrix}
    \end{equation*}

and this equation:

Where matrixes make equation of Ax = b. I want to write corresponding variable, like x, b, A right under the matrixes.

help?

Answer 1

Using \underbrace seems the best approach, but the spacing would be awful. Not to mention the input syntax.

\documentclass{article}
\usepackage{amsmath}
\NewDocumentEnvironment{ubpmatrix}{m+b}
 {% some space to compensate
  \mspace{9mu}%
  % a group to avoid \underbrace being a mathop
  {\underbrace{%
    \mspace{-9mu}%
    \begin{pmatrix}#2\end{pmatrix}%
    \mspace{-9mu}%
  }_{\mathstrut#1}}
  \mspace{9mu}%
 }{}
\begin{document}
[
\begin{ubpmatrix}{A}
    3 & -1 & 1 \
    3 & 3  & 7 \
    3 & 6  & 2
\end{ubpmatrix}
\begin{ubpmatrix}{x}
    x_1 \
    x_2 \
    x_3
\end{ubpmatrix}
=
\begin{ubpmatrix}{b}
    1 \
    4 \
    0
\end{ubpmatrix}
]
\end{document}

The underbrace is below a shrinked object, the same space is added around the construction to cover our tracks.

Answer 2

Here is what you can do with {pNiceMatrix} of nicematrix and its built-in command \SubMatrix.

\documentclass{article}
\usepackage{nicematrix}
\usepackage{lipsum} % for dummy text
\begin{document}
\lipsum[1]
[
A = \begin{pNiceMatrix}[last-row=4]
4  & -1 & 1 \
4  & -8 & 1 \
-2 & 1 & 5 \
\
\CodeAfter \UnderBrace[yshift=1mm]{3-1}{3-3}{A}
\end{pNiceMatrix}
\hspace{2cm}
B = \begin{pNiceMatrix}[last-row=4]
4  & -1 & 1 \
4  & -8 & 1 \
-2 & 1 & 5 \
\
\CodeAfter \UnderBrace[yshift=1mm,shorten]{3-1}{3-3}{B}
\end{pNiceMatrix}
]
\lipsum[2]
\end{document}

The second matrix uses the key shorten of SubMatrix.

Answer 3

The \underbrace macro may be your friend.

\documentclass{article}
\usepackage{amsmath} % for '\underbrace' macro and 'pmatrix' environment
\begin{document}
[
{\underbrace{\begin{pmatrix}
    3 & -1 & 1 \
    3 & 3  & 7 \
    3 & 6  & 2
\end{pmatrix}}{A}} ,
{\underbrace{\begin{pmatrix}
    x_1 \
    x_2 \
    x_3
\end{pmatrix}}{x\vphantom{A}}} = 
{\underbrace{\begin{pmatrix}
    1 \
    4 \
    0
\end{pmatrix}}_{b}}
]
\end{document}