Make equation counter follows customized counter

Question

Objective

Hi. I define a counter, say, no for my customized commands, such as

\newcounter{no}
\counterwithin{no}{chapter}
\newcommand{\no}{\refstepcounter{no}%
                 \theno}
\newcommand{\df}{\normalfont{\textbf{Definition \no}}\;}

Now I would like the equation counter (\theequation) follows the customized counter \thno.

How I use customized counter and what I expect

For example,

\chapter{A chapter}
\begin{itemize}
\item[] \df This is some definition.
\end{itemize}

\begin{equation}
some equation.
\end{equation}

I expect the result would be

Definition 1.1 This is some definition.

                                    some equation.                                 (1.2)

What I've tried

1. I've tried

   \let\theequation\theno
   

   but \theequation does not update in equation environment; the temporary remedy for using equation environment is

   \addtocounter{no}{1}   % update the counter
   \begin{equation}
   ...
   \end{equation}
   

   but that is annoying.

2. Since the customized counter no is NOT used for new environment, remedy using \newtheorem does not fit for me.

3. I've tried

   \let\theequation\theno
   \renewenvironment{equation}{\refstepcounter{no}\begin{equation}}{\end{equation}}
   

   or

   \renewenvironment{equation}{\refstepcounter{no}\begin{equation}}{\tag{\theno}\end{equation}}
   

   but they all give the error message: ! TeX capacity exceeded, sorry [save size=200000].

• By the way, I also wish that the counter of align can follow the customized counter no so that the numbering of no, equation, and algin are consecutive.

Could anyone help? Thanks.

Answer 1

You just want to use equation.

\documentclass{book}

\counterwithin{equation}{chapter}

\newcommand{\no}{\refstepcounter{equation}\theequation}

\newcommand{\df}{\textbf{Definition \no}\enspace\ignorespaces}

\begin{document}

\chapter{A chapter}

\df This is some definition.
\begin{equation}
some equation.
\end{equation}

\end{document}

Note that \normalfont is not a command taking an argument like \textbf. Anyway, I'm not sure why not using standard tools such as \newtheorem.

Better code, in my opinion:

\documentclass{book}
\usepackage{amsthm}

\counterwithin{equation}{chapter}

%\newcommand{\no}{\refstepcounter{equation}\theequation}

\theoremstyle{definition}
\newtheorem{df}[equation]{Definition}

\begin{document}

\chapter{A chapter}

\begin{df}
This is some definition.
\begin{equation}
some equation.
\end{equation}
\end{df}

\end{document}

Answer 2

Is this what you want?

\documentclass{report}
\usepackage{amsmath,amsthm}
\newtheorem{definition}[equation]{Definition}
\begin{document}
\chapter{AAAAA}

\begin{definition}
  hello
\end{definition}

\begin{equation}
  a=b
\end{equation}

\begin{definition}
  world
\end{definition}

\begin{equation}
  c=d
\end{equation}

\begin{definition}
  foo
\end{definition}

\begin{align}
  a & = b \\
    & = c
\end{align}
\end{document}