countersequationsnumbering
I would like to reset equation numbering after each section or subsection. Right now I use
\setcounter{equation}{0}
I saw in another page you could also reset it by adding the following line to the preamble.
\numberwithin{equation}{subsection}
\renewcommand\theequation{\arabic{equation}}
But this doesn't seem to work. The equation counters just increment across subsections. It's the same across sections if we changed the preamble to
\numberwithin{equation}{section}
Here are two ways of doing things, with the help of the changcntr package.
First, if you do not want to display 0for non-existent subsections — i.e. if there is no subsection at one point, display only the section number+the equation number:
\documentclass[11pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amsfonts,amssymb}
\usepackage{chngcntr}
\usepackage{etoolbox}
\counterwithin*{equation}{section}
\counterwithin*{equation}{subsection}
\renewcommand\theequation{\ifnumgreater{\value{subsection}}{0}{\thesubsection.}{\thesection.}\arabic{equation}}%
\begin{document}
\section{A First Section}
\begin{equation}
a = b
\end{equation}
\begin{equation}
a + c = b + c
\end{equation}
\begin{equation}
a × c = b × c
\end{equation}
\section{A Second Section}
\begin{equation}
a = b
\end{equation}
\begin{equation}
a + c = b + c
\end{equation}
\section{A Third Section}
An extra equation :
\begin{equation}\
\ln a = \ln b
\end{equation}
\subsection{First subsection}
\begin{equation}
a = b
\end{equation}
\begin{equation}
a + c = b + c
\end{equation}
\subsection{Second subsection}
\begin{equation}
a = b
\end{equation}
\begin{equation}
a + c = b + c
\end{equation}
\end{document}
If you want the 0 to be displayed, remove from the preamble \renewcommand\theequation{…} and \usepackage{etoolbox}, and replace counterwithin*{equation}{subsection} with the non-starred version.
In addition to the instruction
\numberwithin{equation}{subsection}
you need the instructions
\makeatletter
\@addtoreset{equation}{section}
\makeatother
The first instruction, \numberwithin{equation}{subsection}, resets the equation counter only when a \subsection instruction is encountered. If you have a \section instruction but no \subsection instruction, the counter named equation is not reset unless you provide an additional instruction, such as \@addtoreset{equation}{section}.
Do note that if you do want to number the equations by subsection, it's probably not a good idea to feature equations between a \section directive and that section's first \subsection instruction: at that stage, the subsection counter is still 0 (having been reset to 0 by the \section directive), leading the equation numbers to be formatted as "2.0.1" and quite likely making your readers wonder if the subsection numbered 2.0 has somehow vanished mysteriously.
\documentclass{article}
\usepackage{amsmath}
\numberwithin{equation}{subsection}
\makeatletter
\@addtoreset{equation}{section}
\makeatother
\begin{document}
\section{Section 1 Title}
\subsection{Subsection 1.1 Title}
\begin{equation} a = b \end{equation}
\begin{equation} a+2=b+2 \end{equation}
\section{Section 2 Title}
%%\subsection{Subsection 2.1 Title}
\begin{equation} r=y \end{equation}
\end{document}
Best Answer
Tell LaTeX to reset the equation number whenever
sectionorsubsectionis stepped:The instruction
\counterwithin*is (almost) an interface to\@addtoreset(which is better not used if a higher level command provided by a package is available). With\counterwithin(without*) the counter's representation would be changed, which isn't with the*-form.