在下一行继续下划线

Question

我有以下 MWE：

\documentclass{report}
\usepackage{amsmath}

\begin{document}

\begin{equation} 
\underbrace{\begin{bmatrix}
\Delta \dot{\boldsymbol{x}}_{2,1}\\ 
\Delta \dot{\boldsymbol{x}}_{2,2}\\ 
\vdots \\ 
\Delta \dot{\boldsymbol{x}}_{2,N}
\end{bmatrix}}_{\boldsymbol{y}} = \underbrace{\begin{bmatrix} 
D_2(\boldsymbol{x}_0) & 0 & \cdots & 0 \\ 
0 & D_2(\boldsymbol{x}_1) & \cdots &  0 \\
\vdots & \vdots & \ddots & \vdots \\
0 & 0 & \cdots & D_2(\boldsymbol{x}_{N-1})
\end{bmatrix}\begin{bmatrix}
\Phi^T_{g_{21}}(\boldsymbol{x}_0)\Delta \delta_{e,1} & \Phi^T_{g_{22}}(\boldsymbol{x}_0) \Delta \delta_{a,1} & \Phi^T_{g_{23}}(\boldsymbol{x}_0) \Delta \delta_{r,1} \\ 
\Phi^T_{g_{21}}(\boldsymbol{x}_1)\Delta \delta_{e,2} & \Phi^T_{g_{22}}(\boldsymbol{x}_1) \Delta \delta_{a,2} & \Phi^T_{g_{23}}(\boldsymbol{x}_1) \Delta \delta_{r,2} \\
\vdots & \vdots & \vdots
\\ 
\Phi^T_{g_{21}}(\boldsymbol{x}_{N-1})\Delta \delta_{e,N} & \Phi^T_{g_{22}}(\boldsymbol{x}_{N-1}) \Delta \delta_{a,N} & \Phi^T_{g_{23}}(\boldsymbol{x}_{N-1}) \Delta \delta_{r,N}
\end{bmatrix}}_A \underbrace{\begin{bmatrix}
\boldsymbol{\theta}_{g_{21}} \\ 
\boldsymbol{\theta}_{g_{22}}\\ 
\boldsymbol{\theta}_{g_{23}}
\end{bmatrix}}_{\boldsymbol{\theta}} \,.
\end{equation}

\begin{align} 
\underbrace{\begin{bmatrix}
\Delta \dot{\boldsymbol{x}}_{2,1}\\ 
\Delta \dot{\boldsymbol{x}}_{2,2}\\ 
\vdots \\ 
\Delta \dot{\boldsymbol{x}}_{2,N}
\end{bmatrix}}_{\boldsymbol{y}} &= \begin{bmatrix} 
D_2(\boldsymbol{x}_0) & 0 & \cdots & 0 \\ 
0 & D_2(\boldsymbol{x}_1) & \cdots &  0 \\
\vdots & \vdots & \ddots & \vdots \\
0 & 0 & \cdots & D_2(\boldsymbol{x}_{N-1})
\end{bmatrix} \notag \\
& \qquad \qquad \cdot\begin{bmatrix}
\Phi^T_{g_{21}}(\boldsymbol{x}_0)\Delta \delta_{e,1} & \Phi^T_{g_{22}}(\boldsymbol{x}_0) \Delta \delta_{a,1} & \Phi^T_{g_{23}}(\boldsymbol{x}_0) \Delta \delta_{r,1} \\ 
\Phi^T_{g_{21}}(\boldsymbol{x}_1)\Delta \delta_{e,2} & \Phi^T_{g_{22}}(\boldsymbol{x}_1) \Delta \delta_{a,2} & \Phi^T_{g_{23}}(\boldsymbol{x}_1) \Delta \delta_{r,2} \\
\vdots & \vdots & \vdots
\\ 
\Phi^T_{g_{21}}(\boldsymbol{x}_{N-1})\Delta \delta_{e,N} & \Phi^T_{g_{22}}(\boldsymbol{x}_{N-1}) \Delta \delta_{a,N} & \Phi^T_{g_{23}}(\boldsymbol{x}_{N-1}) \Delta \delta_{r,N}
\end{bmatrix} \underbrace{\begin{bmatrix}
\boldsymbol{\theta}_{g_{21}} \\ 
\boldsymbol{\theta}_{g_{22}}\\ 
\boldsymbol{\theta}_{g_{23}}
\end{bmatrix}}_{\boldsymbol{\theta}} \,.
\end{align}


\end{document}

结果如下：

因为这个公式太长，一行写不完，所以我需要分成两行。但是现在，我该如何添加下括号A。

Answer 1

我不确定你想要什么。

\documentclass{scrreprt}
\usepackage{mathtools}
\usepackage{etoolbox}

\makeatletter
\let\lunderbrace\underbrace
\let\runderbrace\underbrace
\let\lupbracefill\upbracefill
\let\rupbracefill\upbracefill
\patchcmd{\lunderbrace}{\upbracefill}{\lupbracefill}{}{}
\patchcmd{\runderbrace}{\upbracefill}{\rupbracefill}{}{}
\patchcmd{\lupbracefill}{\braceru}{\leaders\vrule\@height\ht\z@\@depth\z@\hskip\wd\z@}{}{}
\patchcmd{\rupbracefill}{\bracelu}{\leaders\vrule\@height\ht\z@\@depth\z@\hskip\wd\z@}{}{}
\makeatother

\begin{document}

\begin{equation}
  \lunderbrace{abcdefghijklm}_{2} = \runderbrace{abcdefghijklmn}_{2}
\end{equation}

\end{document}