3

I'm trying to write a set of equations in \begin{cases}. The i-th equation should have the tag P.i. I can use \tag{P} to number the entire set as P, or, I can use \begin{subequations} to number the i-th equation as 1.i. But I can't number my equations as P.1. I tried the following:

\begin{subequations}\begin{align}\label{eq:a}\tag{P}
    \begin{cases}
       a &a \tag{\ref{a}}\\
       a &a.
    \end{cases}
    \end{align}
\end{subequations}

but it doesn't work. Does anyone have a suggestion?

  • Welcome to tex.sx. Perhaps off-topic: if there is only one cases element, what's the reason to use align? equation would be better for that. – barbara beeton Nov 7 '19 at 19:00
1

Something like this?

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{subequations}\begin{align}\label{eq:a}
    \begin{cases}
       a &a \tag{P.\theequation}\\
       a &a.
    \end{cases}
    \end{align}
\end{subequations}
In \ref{eq:a}, we find blah-blah
\end{document}

enter image description here

1

Ued subnumcases environment from cases for getting subnumbering for case.
Used @egreg's answer to the question: "subequations customize arabic numbering in aligned environment" for changing the equation numbering to arabic.

\documentclass[10pt]{article}
\usepackage{amsmath,etoolbox}
\usepackage{cases}

\patchcmd\subequations
 {\theparentequation\alph{equation}}
 {\subequationsformat}
 {}{}


\newcommand{\subequationsformat}{\theparentequation.\arabic{equation}}
\begin{document}
\renewcommand{\theequation}{P}
   \begin{subnumcases}{}
       a &a \label{a}\\
       b &b \label{b}
    \end{subnumcases}

\renewcommand{\theequation}{Q}
   \begin{subnumcases}{}
       c &c \label{c}\\
       d &d \label{d}
    \end{subnumcases}    

    Refer to \ref{a}, \ref{b}, \ref{c} or \ref{d}.
\end{document}

enter image description here

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