\documentclass{article}%
%
\usepackage{graphicx}%
\usepackage{mathptmx}%
\usepackage{subcaption}%
\usepackage{amsmath}%
\usepackage{booktabs}%
\usepackage{amsthm}%
\usepackage{amssymb}%
%
\begin{document}
\begin{figure}
\begin{subfigure}{.55\textwidth}
\centering
\includegraphics[height=6.6cm]{example-image-a}
\subcaption{Exact solution}
\end{subfigure}
\begin{subfigure}{.55\textwidth}
\centering
\includegraphics[height=6.6cm]{example-image-a}
\subcaption{N=2}
\end{subfigure}
\begin{subfigure}{.55\textwidth}
\centering
\includegraphics[height=6.6cm]{example-image-a}
\subcaption{N=4}
\end{subfigure}
\begin{subfigure}{.45\textwidth}
\centering
\includegraphics[height=5.5cm]{example-image-a}
\subcaption{t=0.5}
\end{subfigure}
\caption{Exact and discretized solutions for N=2 and 4. Exact and estimated solutions with t=0.5 for Example 1.}
\end{figure}
\end{document}
答案1
您的文档出现多种形式的错误
! Package inputenc Error: Unicode character (U+200E)
因为它在几个地方都有 Unicode 从右到左的控制标记。
最好删除它们，但你可以添加
\DeclareUnicodeCharacter{200E}{}
到文件顶部，这样该字符就不执行任何操作，然后您的文档运行时不会出错，尽管会警告这些示例宽度太宽，但您的实际图像可能大小不同，所以我忽略了这一点。另外，我不知道您在问题标题中提到的“偶数”是哪个。
\DeclareUnicodeCharacter{200E}{}
\documentclass{article}%
%
\usepackage{graphicx}%
\usepackage{mathptmx}%
\usepackage{subcaption}%
\usepackage{amsmath}%
\usepackage{booktabs}%
\usepackage{amsthm}%
\usepackage{amssymb}%
%
\begin{document}
\begin{figure}
\begin{subfigure}{.55\textwidth}
\centering
\includegraphics[height=6.6cm]{example-image-a}
\subcaption{Exact solution}
\end{subfigure}
\begin{subfigure}{.55\textwidth}
\centering
\includegraphics[height=6.6cm]{example-image-a}
\subcaption{N=2}
\end{subfigure}
\begin{subfigure}{.55\textwidth}
\centering
\includegraphics[height=6.6cm]{example-image-a}
\subcaption{N=4}
\end{subfigure}
\begin{subfigure}{.45\textwidth}
\centering
\includegraphics[height=5.5cm]{example-image-a}
\subcaption{t=0.5}
\end{subfigure}
\caption{Exact and discretized solutions for N=2 and 4. Exact and estimated solutions with t=0.5 for Example 1.}
\end{figure}
\end{document}
