DFT实现线性卷积

\[\begin{align*} \newcommand{\dif}{\mathop{}\!\mathrm{d}} \newcommand{\belowarrow}[1]{\mathop{#1}\limits_{\uparrow}} \newcommand{\bd}{\boldsymbol} \newcommand{\L}{\mathscr{L}} \newcommand{\red}{\color{red}} \newcommand{\xleftrightarrow}[1]{\stackrel{#1}{\longleftrightarrow}} \end{align*}\]

(没时间写,等考完试再补上,如果我忘了的话,请提醒一下我)