$$
\def\E{\mathbb{E}} %expectation
\def\P{\mathbb{P}} %prob
\def\var{\mathbb{V}} %var
\def\T{\mathrm{\scriptscriptstyle T}} %transpose
\def\ind{\mathbbm{1}} %indicator
\newcommand{\gw}[1]{\color{red}{(#1)}}
\newcommand{\norm}[1]{\lVert#1\rVert}
\newcommand{\abs}[1]{\lvert#1\rvert}
$$
We introduce the convergence results of $k$-NN including the optimal convergence rate.