Build a matrix with overbrace on some columns

Question

I need to create a matrix and some overbraces on some entries at the top of the matrix like the following image(the entries are omitted)

I tried this code

$$  
A_n = \begin{bmatrix}
a_1&b_{1,2}&\overbrace{0, \cdots, 0}^{N_1-1}&b_{1,S_1+1}&\overbrace{0, \cdots, 0}^{N_2-1}
\end{bmatrix}
$$

and this was the result. I don't want the overbrace to be inside the matrix

How can I do that?

Answer 1

May be this will help you (adapted from this answer):

\documentclass{article}
\usepackage{amsmath}
\newcommand\bovermat[2]{%
    \makebox[0pt][l]{$\smash{\overbrace{\phantom{%
                    \begin{matrix}#2\end{matrix}}}^{\text{#1}}}$}#2}
\newcommand\partialphantom{\vphantom{\frac{\partial e_{P,M}}{\partial w_{1,1}}}}
\begin{document}
\begin{equation}
\begin{matrix}
    A_n
    =
    \begin{bmatrix}
    a_1&b_{1,2}&\bovermat{1}{0,\cdots,0} & b_{1,s_{1}+1} &
    \bovermat{2}{0,\cdots,0}\\
    \end{bmatrix}
    \end{matrix}
    \end{equation}
\end{document}

which gives: