I want to write an optimization problem wherein I want the "number" to be something like "PP".
The code below achieves this in the align environment.
\begin{aligned}
& \underset{x}{\text{minimize}}
& & f_0(x) \label{Eq:PP} \tag{PP} \\
& \text{subject to}
& & f_i(x) \leq b_i, \; i = 1, \ldots, m.
\end{aligned}
\end{align}
However, doing that produces (PP) on the first line of the equation. I can use
\begin{aligned}
& \underset{x}{\text{minimize}}
& & f_0(x) \label{Eq:PP}\\
& \text{subject to}
& & f_i(x) \leq b_i, \; i = 1, \ldots, m.
\end{aligned}
\end{equation}
Here, the number appears in the center, as it should, but if I try "tag{PP}", I get an error. I was wondering if there's a way to write the optimization problem so that the number "PP" appears is vertically at the enter of the problem, and not aligned with the first line.
Thanks!