%\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}
![bb572fcae72004aecc3aa1c5c906bbeb.jpg](https://pics.latexstudio.net/uploads/20250612/bb572fcae72004aecc3aa1c5c906bbeb.jpg)

documentclass[twocolumn]{IEEEtran}
usepackage{mathrsfs}
usepackage{color}
usepackage{cite}
usepackage{flafter}
usepackage{epsfig}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amssymb}
usepackage{empheq}
usepackage{graphicx}
usepackage{tabularx} %
usepackage{booktabs} %
usepackage{caption}
usepackage{float}
usepackage[font={small,rm}]{caption}
DeclareGraphicsRule{.jpg}{eps}{.bb}{}
newtheorem{theorem}{Theorem}
newtheorem{corollary}{Corollary}
newtheorem{definition}{Definition}
newtheorem{lemma}{Lemma}
newtheorem{proposition}{Proposition}
newtheorem{algorithm}{Algorithm}
newtheorem{assumption}{Assumption}
newtheorem{remark}{Remark}
renewcommand{baselinestretch}{1.0}
allowdisplaybreaks

begin{document}

textbf{OP5:}
begin{subequations}label{op5}
begin{empheq}
[left={OP5:empheqlbrace}]{align}
&underset{u(k)}{min}~ psi label{op51}\
s.t. ~~&J_{infty}(k) leq psi label{op52}\
&xi(k+1|k)in Phi_{M+1} label{op53} \
&underset{p}{max}left|left[u(k)right]_pright|leq bar{u}, pin {1,ldots,n_u} label{op54} \
&underset{q}{max}left|left[x(k)right]_qright|leq bar{x}, qin {1,ldots,n_x} label{op55}
end{empheq}
end{subequations}
end{document}