How can I place two equations on one line, but each is numbered separately?

Question

I'd like to place two equations on one line, but each has to be numbered separately.

For example, in this figure:

This figure is created with minipage:

\noindent\begin{minipage}{.5\linewidth}
\begin{equation}
\varepsilon_k = \lfloor \dfrac{\varepsilon}{K} \rfloor,
\end{equation}
\end{minipage}%
\begin{minipage}{.5\linewidth}
\begin{equation}
\mathbb{P}_k = \dfrac{\mathbb{P}}{k}.
\end{equation}
\end{minipage}

But I don't like using this solution, because it seems to be unprofessional (e.g, it occupies the blank space before and after equations).

I prefer using align, like this:

\begin{align}
& \varepsilon_k = \dfrac{\varepsilon}{K}
& \mathbb{P}_k = \dfrac{\mathbb{P}}{k}
\end{align}

This yields:

But how can I make these two equations be numbered separately?

Answer 1

I wouldn't use such an approach, which makes for difficult reading.

Anyway…

\documentclass{article}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{lipsum} % for mock text

\ExplSyntaxOn
\NewDocumentEnvironment{multiequation}{b}
 {
  \vantiempham:n { #1 }
 }
 {}

\seq_new:N \l__vantiempham_md_seq

\cs_new_protected:Nn \vantiempham:n
 {
  \seq_set_split:Nnn \l__vantiempham_md_seq { \\ } { #1 }
  $$
  \seq_map_function:NN \l__vantiempham_md_seq \__vantiempham_md_item:n
  $$
 }
\cs_new_protected:Nn \__vantiempham_md_item:n
 {
  \begin{minipage}{\dim_eval:n {\displaywidth/(\seq_count:N \l__vantiempham_md_seq)} }
  \begin{equation}
  \cs_set_eq:Nc \label { ltx@label }
  #1
  \vphantom{\seq_use:Nn \l__vantiempham_md_seq {}}
  \end{equation}
  \end{minipage}
 }

\ExplSyntaxOff  

\begin{document}

\eqref{A}, \eqref{B}, \eqref{C}, \eqref{D}, \eqref{E}

\lipsum[1][1-4]
\begin{multiequation}
  \varepsilon_k = \left\lfloor\frac{\varepsilon}{K}\right\rfloor,
  \label{A}
\\
  \mathbb{P}_k = \frac{\mathbb{P}}{k}.
  \label{B}
\end{multiequation}
\lipsum[2][1-4] % filler text
\begin{multiequation}
  \varepsilon_k = \left\lfloor\frac{\varepsilon}{K}\right\rfloor,
  \label{C}
\\
  \mathbb{P}_k = \frac{\mathbb{P}}{k},
  \label{D}
\\
  a=b.
  \label{E}
\end{multiequation}
\lipsum[3][1-4]

\end{document}

Equations are separated by the standard \\ (in this case it doesn't mean “new line”).

Note that the equation numbers are aligned, because in each of the items we typeset a vertical phantom of all of them, No manual adjustment, contrary to what happens with Mico's answer.

Answer 2

If your main concern is having enough vertical whitespace padding above and below the equations, I suggest you place the side-by-side minipage environments in a center environment.

\documentclass{article}

\usepackage{mathtools,amssymb,lipsum}
\DeclarePairedDelimiter{\floor}{\lfloor}{\rfloor}

\begin{document}
\lipsum[1][1-4] % filler text
\begin{center}
\begin{minipage}{.5\linewidth}
  \begin{equation}
  \varepsilon_k = \floor*{\frac{\varepsilon}{K}}\,,
  \end{equation}
\end{minipage}%
\begin{minipage}{.5\linewidth}
  \begin{equation}
  \mathbb{P}_k = \frac{\mathbb{P}}{k}\,.
  \end{equation}
\end{minipage}
\end{center}
\lipsum[2][1-4] % filler text
\end{document}

---

Addendum: If the formulas in the two minipage environments have different heights and/or depths, the solution suggested above is not guaranteed to align the equation numbers perfectly. Luckily, this problem can be easily taken care of by inserting a suitable typographical strut -- created, in practice, with a \vphantom directive -- in the equation that's less tall and/or less deep.

Take the two equations above: Visual inspection reveals that the one on the right (\frac{\mathbb{P}}{k}) is both taller and -- impportantly -- deeper than the one on the left. It therefore suffices to change the

  \begin{equation}
  \varepsilon_k = \floor*{\frac{\varepsilon}{K}}\,,
  \end{equation}

to

  \begin{equation}
  \varepsilon_K = \floor*{\frac{\varepsilon}{K}}
     \vphantom{\frac{\mathbb{P}}{k}}\,,
  \end{equation}

to obtain the desired even alignment: