1、Latex中将字母上下方插入字母数字\mathop{a}\limits_{i=1}：
$\underset{i=1}{a}$

2Latex罗马数字
大写：\uppercase\expandafter{\romannumeral20}
小写：\romannumeral20

2、LaTex一行排列多个图，并且加入每个图都添加小标题

效果：

代码：

\begin{figure}[!ht]\centering\begin{subfigure}[b]{0.99\textwidth}\fontsize{10.0pt}{\baselineskip}\selectfont\rotatebox{90}{~~~~~~~~[29]}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc101.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc102.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc103.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc104.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc105.pdf}\end{minipage}%\caption{}\label{fig:3-11-1V}\end{subfigure}%\\\begin{subfigure}[b]{0.99\textwidth}\fontsize{10.0pt}{\baselineskip}\selectfont\rotatebox{90}{~~~~~~~~[29]}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc101.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc102.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc103.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc104.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc105.pdf}\end{minipage}%\caption{}\label{fig:3-11-2Lap}\end{subfigure}\\\begin{subfigure}[b]{0.99\textwidth}\fontsize{10.0pt}{\baselineskip}\selectfont\rotatebox{90}{~~~~~~~~[29]}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc101.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc102.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc103.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc104.pdf}\end{minipage}\begin{minipage}[t]{0.185\linewidth}\centering\includegraphics[width=1\linewidth]{pic3/voc105.pdf}\end{minipage}%\caption{}\label{fig:3-11-3Fillet}\end{subfigure}%\bicaption{不同类型的焊接工件。(a) 对接接头，(b) 搭接接头，(c) 角接接头。}{Different types of welding workpieces. (a) Butt joint, (b) Lap joint, (c) Fillet joint.}\label{fig:weldingworkpiece}
\end{figure}

3、效果

代码：

\begin{figure}[!ht]\centering\begin{subfigure}[b]{0.23\textwidth}%控制sufigure宽度\begin{minipage}[t]{0.9\linewidth}\includegraphics[width=1\linewidth]{pic3/voc101.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc102.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc103.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc104.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc105.pdf}\end{minipage}\caption{}\label{fig:3-11-1V}\end{subfigure}%\begin{subfigure}[b]{0.23\textwidth}\begin{minipage}[t]{0.9\linewidth}\includegraphics[width=1\linewidth]{pic3/voc101.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc102.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc103.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc104.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc105.pdf}\end{minipage}\caption{}\label{fig:3-11-2Lap}\end{subfigure}\begin{subfigure}[b]{0.23\textwidth}\begin{minipage}[t]{0.9\linewidth}\includegraphics[width=1\linewidth]{pic3/voc101.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc102.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc103.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc104.pdf}\vspace{4pt}\includegraphics[width=1\linewidth]{pic3/voc105.pdf}\end{minipage}\caption{}\label{fig:3-11-3Fillet}\end{subfigure}%\bicaption{不同类型的焊接工件。(a) 对接接头，(b) 搭接接头，(c) 角接接头。}{Different types of welding workpieces. (a) Butt joint, (b) Lap joint, (c) Fillet joint.}\label{fig:weldingworkpiece}
\end{figure}

参考文献：
https://blog.csdn.net/qq_37424778/article/details/120882254