%\usepackage{cases}
\begin{numcases}{\text{OP3}}
\quad ~~a, & if $n = 0$ \label{eq:numcase1}\tag{41a}\\
\text{s.t.}~ b, & if $x\in(0,1)$ \label{eq:numcase2}\tag{41b}\\
\quad ~~a, & if $n = 0$ \label{eq:numcase3}\tag{41c}\\
\quad ~~a, & if $n = 0$ \label{eq:numcase4}\tag{41d}
\end{numcases}

问 想要输入左端的大括号,而且每个公式要有子编号?