I want to draw 2 braces under my matrix to indicate that the vectors a are different from the vectors x and explain why. Nevertheless, my current attempt just draws the brace in the middle of my matrice... Does someones has an idea, please? I try to add a snippet here, my code is also here: https://www.overleaf.com/read/nnkpkdrwphmd
Thank you so much for your help!
\documentclass{article}
\usepackage{nicematrix}
\usepackage{tikz}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usetikzlibrary{patterns}
\usetikzlibrary{matrix,decorations.pathreplacing}
\usetikzlibrary{decorations.pathreplacing,calligraphy}
\title{Fairness}
\author{someone}
\date{May 2021}
\begin{document}
\maketitle
\section{Introduction}
\NiceMatrixOptions{code-for-first-row = \color{red},
code-for-first-col = \color{blue},
code-for-last-row = \color{green},
code-for-last-col = \color{magenta}}
$\begin{pNiceMatrix}%
[margin,
first-row,
first-col,
%code-for-first-row = \mathbf{\alph{jCol}} ,
%code-for-first-col = \mathbf{\arabic{iRow}} ,
nullify-dots,
xdots/line-style=loosely dotted,
code-after = {\begin{tikzpicture}[decoration={calligraphic brace, mirror, amplitude=6pt,raise=2pt}]
\draw[decorate,thick] (0-1.south south) -- (1-3.south east);
node[midway,under=1.5ex]{$\scriptstyle\mathcal{A}_N^i$};
\end{tikzpicture}}
]
& 1 & 2 & \Cdots & k & k+1 & k+2 & \Cdots & m \\
1 & a_{(1,1)} & a_{(1,2)} & \Cdots & a_{(1,k)} & x_{(1,k+1)} & x_{(1,k+2)} & \Cdots & x_{(1,m)} \\
2 & a_{(2,1)} & a_{(2,2)} & \Cdots & a_{(2,k)} & x_{(2,k+1)} & x_{(2,k+2)} & \Cdots & x_{(2,m)} \\
3 & a_{(3,1)} & a_{(3,2)} & \Cdots & a_{(3,k)} & x_{(3,k+1)} & x_{(3,k+2)} & \Cdots & x_{(3,m)} \\
\Vdots & \Vdots & \Ddots & \Cdots & \Vdots & \Vdots & \Ddots & \Cdots & \Vdots \\
& & & \Cdots & & & & \Cdots & & \\
n & a_{(n,1)} & a_{(n,2)} & \Cdots & a_{(n,k)} & x_{(n,k+1)} & x_{(n,k+2)} & \Cdots & x_{(n,m)}
%\CodeAfter \SubMatrix{\{}{1-1}{3-1}{.}[left-xshift=0.5em]
\end{pNiceMatrix}$
\end{document}
