答案1
最后一点不清楚(???应该是......),但其余部分似乎可以遵循:
\documentclass{article}
\usepackage{tikz}
\usepackage{amsmath}
\begin{document}
\begin{tikzpicture}
\draw[very thick] (2,2)--node[above]{$\mathcal{X}$}(-2,2)--node[left]{$\mathcal{X}$}(-2,-2)--(2,-2)--cycle;
\draw[dashed](-2,2)--(2,-2);
\draw[<->] (-0.5,-0.5)--node[above]{$\varepsilon$}(0.5,0.5);
\end{tikzpicture}
\end{document}