# About special numbering in equation

I would like to ask how to typeset the equation number like Charles's law in the picture? There is a \circ after the closing parenthersis. Do I need another package to do that? The packages that I am using are chemfig, mhchem and graphicx. Thanks a lot.

I am sorry that I cannot figure it out after reading Custom equation numbering. I would like to have the equation numbering like it in the picture.

\newcommand*{\myTagFormat}[2]{(\ref{#1})($#2$)}

\begin{flushleft}
Boyle's law
\end{flushleft}

\begin{align}
\refstepcounter{equation}\label{eq:base}
pV &= constant,\text{ }at\text{ }constant\text{ }n, T\\
\tag*{\myTagFormat{eq:base}{n}}\label{eq:base-n}
\end{align}


I'd like to suggest that you load the mathtools package -- a superset of the amsmath package -- and use its \newtagform and \usetagform macros. (To revert to the default equation tags, run \usetagform{default}.)

\documentclass{article} % or some other suitable document class

\usepackage[fleqn]{mathtools}
\newtagform{Charles}{$($}{$)^{\circ}$}
\usetagform{Charles}
\numberwithin{equation}{section} % just for this example

\begin{document}
\stepcounter{section}
\setcounter{equation}{4} % just for this example

\noindent We assume that \dots
\begin{gather}
\textup{\bfseries Boyle's law: } pV=\dots \\
\textup{\bfseries Charles's law: } V = \dots \\
\hphantom{\textup{\bfseries Charles's law: }} p = \dots \\
\textup{Avogadro's principle: } V = \dots
\end{gather}
Boyle's and Charles's laws \dots
\end{document}

• Thanks so much. I would like to ask if there is code that only affect only 1 equation at a time but not the whole equation? As I still have some other equation that does not have \circ in the numbering. Jan 8 at 11:45
• @someone - Not sure I understand "affect only 1 equation at a time but not the whole equation". (Did you mean to write "...not the whole document"?) You could run \usetagform{Charles} and \usetagform{default} right before and after the "circled" equation in order to limit the scope of the "charles" tag form tp just that equation.
– Mico
Jan 8 at 11:49