writing bracket under only block vector matrix

Question

I write column vector using \bmatrix then I want to write underbrace and add text in this brace. but I don't how to do this. My code is

\documentclass
[a4paper, 12pt, oneside]{Thesis} 
\usepackage{amsmath,amssymb,amstext,amsfonts}
\begin{document}
     \begin{align*}
    B&=
     \begin{pmatrix}
     \begin{bmatrix}
     1 \\           
     2 \\
     3\\
     \underbrace{b_{1}}
     \end{bmatrix} 
    \end{pmatix}
    \end{align*}
\end{document}

The bracket is written in column vector but I want to put bracket and b_{1} outside the column vector. Please help me in doing this

Answer 1

I guess, what you want is one of these (see also here for more complicated structures):

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
B&=
\underbrace{%
\begin{pmatrix}
 \begin{bmatrix}
 1 \\           
 2 \\
 3 \\
 \end{bmatrix} 
\end{pmatrix}%
}_{b_{1}}
\end{align}

\begin{align}
B&=
\begin{pmatrix}
  \underbrace{%
    \begin{bmatrix}
    1 \\           
    2 \\
    3 \\
    \end{bmatrix}%
  }_{b_{1}}
\end{pmatrix}
\end{align}

\end{document}