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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}