目前我有一组使用互文在它们之间嵌套文本的方程式，但我遇到最后一个方程式拒绝输出的问题（等式：4.3）
\documentclass[10pt]{article}
\usepackage{amsmath}
\setlength{\parskip}{1em}
\setlength{\parindent}{0pt}
\begin{document}
hello123
\begin{subequations}
\renewcommand{\theequation}{\theparentequation.\arabic{equation}}
\begin{align}
\left ( \frac{\partial \rho }{\partial t}+u_{i}\frac{\partial \rho }{\partial x_{j}}\right )+\rho \frac{\partial u_{i} }{\partial x_{i}}&=0 \label{eq:1.1}\\
\rho\left(\frac{\partial u_{i}}{\partial t}+u_{i}\frac{\partial u_{i}}{\partial x_{j}}\right)+\frac{\partial P}{\partial x_{j}}&=\mu\left(\frac{\partial^2 u_{i}}{\partial x_{j}^2}\right)+\rho g \label{eq:1.2}
\end{align}
\end{subequations}
hello123
\begin{subequations}
\renewcommand{\theequation}{\theparentequation.\arabic{equation}}
\begin{align}
u\left(\overrightarrow{r}\right)=\lim_{T \rightarrow \infty}\left(\frac{1}{T}\int_{t}^{t+T} u(\overrightarrow{r},t)dt\right)\label{eq:2.1}
\intertext{hello123}
\left(\overrightarrow{r},t\right)=\lim_{V \rightarrow \infty}\left(\frac{1}{V}\iiint u(\overrightarrow{r},t)dt\right)\label{eq:2.2}
\intertext{hello123}
u\left(\overrightarrow{r},t\right)=\lim_{V \rightarrow \infty}\frac{1}{M}\sum_{m=1}^Mu_n\left(\overrightarrow{r},t\right)\label{eq:2.3}
\end{align}
\end{subequations}
hello123
\par
hello123
\begin{subequations}
\renewcommand{\theequation}{\theparentequation.\arabic{equation}}
\begin{align}
u&=\overline{u}+u' \label{eq:3.1}\\
P&=\overline{P}+P' \label{eq:3.2}
\end{align}
\end{subequations}
hello123
\begin{subequations}
\renewcommand{\theequation}{\theparentequation.\arabic{equation}}
\begin{align}
\rho\left(\frac{\partial\left(\overline{ u_{i}}+u_i'\right)}{\partial t}+\left(\overline u_{i}+u_i'\right)\frac{\partial \left(\overline{ u_{i}}+u_i'\right)}{\partial x_{j}}\right)+\frac{\partial \left(\overline P+P'\right)}{\partial x_{j}}&=\mu\left(\frac{\partial^2 \left(\overline u_{i}+u_{i}'\right)}{\partial x_{j}^2}\right)+\rho g \label{eq:4.1}
\intertext{hello123}
\rho\left(\frac{\partial\left(\overline{ u_{i}}\right)}{\partial t}+\overline u_{i}\frac{\partial \left(\overline{ u_{i}}\right)}{\partial x_{j}}\right)+\frac{\partial \left(\overline P\right)}{\partial x_{j}}&=\mu\left(\frac{\partial^2 \left(\overline u_{i}\right)}{\partial x_{j}^2}\right)+\rho g \label{eq:4.2}
\intertext{hello123}
\rho\left(\frac{\partial\langle u_{i}\rangle}{\partial t}+\left(\langle u_{i}\rangle\langle u_{j}\rangle\right)\frac{\partial}{\partial x_{j}}+\langle u_{i}'u_{j}'\rangle\frac{\partial}{\partial x_{j}}\right )+\frac{\partial \langle P\rangle}{\partial x_{j}}=\mu\triangledown^2\langle u_{i}\rangle-\frac{\partial \langle\tau\rangle}{\partial x_{j}}\label{eq4.3}
\end{align}
\end{subequations}
\end{document}
我收到第 45 行的以下错误消息
\end{align}
即使没有 eq:4.3，代码也能完美运行，所以我认为这与方程的长度无法正确对齐有关？
答案1
您收到的错误表明宏（控制序列）\triangledown未定义。从您的屏幕截图中看不到，但如果您打开文件.log，您会看到它说
! Undefined control sequence.
<argument> ...}{\partial x_{j}}=\mu \triangledown
^2\langle u_{i}\rangle -\f...
l.51 \end{align}
?
在这种情况下，换行前的最后一个宏是未定义/未知的宏\triangledown。
添加
\usepackage{amssymb}
要解决这个问题。