Giving a text number to an equation in the equation mode

Question

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!

Answer 1

The label and tag had to be outside of aligned environment:

\documentclass{article}
\usepackage{amsmath}
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{equation}
\setlength\PreviewBorder{1em}

\begin{document}
    \begin{equation} \label{Eq:PP} \tag{PP} 
\begin{aligned}
    & \underset{x}{\text{minimize}}
        & & f_0(x) \\
    & \text{subject to}
        & & f_i(x) \leq b_i, \; i = 1, \ldots, m.
\end{aligned}
    \end{equation}
\end{document}

Answer 2

I use another approach using optidef package that it is a library for optimization problems. The inconvenience of this package, IMHO, are the labels (in fact I have not able to put the label into the two equation or it is impossible).

\documentclass[a4paper,12pt]{article}

\usepackage{mathtools,amssymb}
\usepackage{optidef}
\begin{document}

\begin{mini!}
{x}{f_0(x)}{}{}\notag
\addConstraint{f_i(x) }{\leq b_i,\quad}{ i = 1, \ldots, m.}\tag{PP}
\end{mini!}

\end{document}