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答案1
从未见过这种公式,所以我不知道这是否正是你想要实现的:
\documentclass{article}
\usepackage{amsmath}
\newcommand{\mymatr}[2]{\big[\overset{\scriptscriptstyle #1}{#2}\big]}
\newcommand{\myind}[1]{\raisebox{1pt}{$\scriptscriptstyle #1$}}
\begin{document}
\[
\mymatr{N}{X}\myind{M}=
\myind{M}\mymatr{M}{U}\mymatr{N}{S}\myind{M}
\mymatr{N}{V}^{T}\myind{N}
\]
or
\[
\mymatr{N}{X}\myind{M}=
\myind{M}\mymatr{M}{U}\mymatr{N}{S}\myind{M}
\mymatr{N}{V}^{\scriptscriptstyle T}\myind{N}
\]
\end{document}