An elegant aspect for enumerate the equations of a book

Question

I am trying to create this kind of enumeration for the various equations of my book, similar, as this image:

My MWE is:

\documentclass{book}
\usepackage{mathtools}
\usepackage{newtxtext}
\usepackage{newtxmath}
\PassOptionsToPackage{svgnames, dvipsnames, table, x11names}{xcolor}
\usepackage{tikz}
\usepackage{classico}
\usepackage{pifont}
\definecolor{ocre}{RGB}{243,102,25} 
%----------------------------------------------------------------------------------
%   Change number equations with sections
%----------------------------------------------------------------------------------
\usepackage{chngcntr}
\counterwithin{equation}{section}
\renewcommand{\theequation}{\bfseries{\sffamily{\thesection-\arabic{equation}}}\textcolor{ocre}{\ding{228}}}
\begin{document}
\begin{equation}
\oint \mathbf{E} \cdot d \mathbf{a} = \int \rho \, dv 
\end{equation}
\begin{equation}
\oint \mathbf{B} \cdot d \mathbf{a} =  \mathbf{0} 
\end{equation}
\end{document}

with this output:

I have encountered the following problems (see the image below):

1. I don't know how to eliminate the round brackets;
2. if we suppose we don't delete the round brackets, the one on the left is not in bold while the one on the right is in bold.
3. Using the pifont package I was trying to reverse the direction of the arrow to the left using \rotatebox{...}{...}, with the graphicx package, but I am not, probably, to rotate the direction of the arrow to the left.

Answer 1

The following produces what you're after:

\documentclass{book}

\usepackage{mathtools}
\usepackage[svgnames, dvipsnames, table, x11names]{xcolor}
\usepackage{pifont}

\definecolor{ocre}{RGB}{243,102,25} 
%----------------------------------------------------------------------------------
%   Change number equations with sections
%----------------------------------------------------------------------------------
\usepackage{chngcntr}
\counterwithin{equation}{section}

\renewcommand{\theequation}{\thesection-\arabic{equation}}
\newcommand{\eqnnumsymbol}{\textcolor{ocre}{\reflectbox{\ding{228}}}}

\makeatletter
\newtagform{bbrackets}% <name>
  [\textbf]% Internal style
  {(}% Left bracket
  {\ifnum\pdfstrcmp{\@currenvir}{equation}=0
     \eqnnumsymbol%
   \else
     \ifnum\pdfstrcmp{\@currenvir}{align}=0
       \eqnnumsymbol%
     \fi
   \fi
   )}% Right bracket
\makeatother
\usetagform{bbrackets}

\begin{document}

See both~\eqref{eq:eqn1} and~\eqref{eq:eqn2}. Also see~\eqref{eq:eqn3}.

\begin{equation}
  \oint \mathbf{E} \cdot d \mathbf{a} = \int \rho \, \mathrm{d}v \label{eq:eqn1}
\end{equation}

\begin{equation}
  \oint \mathbf{B} \cdot d \mathbf{a} =  \mathbf{0} \label{eq:eqn2}
\end{equation}

\begin{align}
  abc \label{eq:eqn3}
\end{align}

\end{document}

It uses

• mathtools' \newtagform to establish a new form of tag for equations and related environments.

• A conditional to set the symbol. If inside an equation or align, the symbol is added, otherwise it's left out. This allows you to use \eqref. It's not extensively tested, but works in the example provided.

---

If you want to remove the bold numbering from your reference, you can use the following:

\documentclass{book}

\usepackage{mathtools}
\usepackage[svgnames, dvipsnames, table, x11names]{xcolor}
\usepackage{pifont}

\definecolor{ocre}{RGB}{243,102,25} 
%----------------------------------------------------------------------------------
%   Change number equations with sections
%----------------------------------------------------------------------------------
\usepackage{chngcntr}
\counterwithin{equation}{section}

\makeatletter

\renewcommand{\theequation}{\thesection-\arabic{equation}}
\newcommand{\eqnumfmt}{%
  \ifnum\pdfstrcmp{\@currenvir}{equation}=0
    \bfseries
  \else
    \ifnum\pdfstrcmp{\@currenvir}{align}=0
      \bfseries
    \fi
  \fi}
\newcommand{\eqnnumsymbol}{\textcolor{ocre}{\reflectbox{\ding{228}}}}

\newtagform{bbrackets}% <name>
  [\eqnumfmt]% Internal style
  {(\bgroup}% Left bracket
  {\ifnum\pdfstrcmp{\@currenvir}{equation}=0
     \eqnnumsymbol%
   \else
     \ifnum\pdfstrcmp{\@currenvir}{align}=0
       \eqnnumsymbol%
     \fi
   \fi
   \egroup)}% Right bracket
\makeatother
\usetagform{bbrackets}

\begin{document}

See both~\eqref{eq:eqn1} and~\eqref{eq:eqn2}. Also see~\eqref{eq:eqn3}.

\begin{equation}
  \oint \mathbf{E} \cdot d \mathbf{a} = \int \rho \, \mathrm{d}v \label{eq:eqn1}
\end{equation}

\begin{equation}
  \oint \mathbf{B} \cdot d \mathbf{a} =  \mathbf{0} \label{eq:eqn2}
\end{equation}

\begin{align}
  abc \label{eq:eqn3}
\end{align}

\end{document}

Answer 2

I propose this other solution, without parentheses, based on the \newtagform command from mathtools and the adforn ornaments package, which has a left-oriented arrow tip. I took the liberty to change its colour, which suits better the black characters (quia gustum meum…). Of course, feel free to choose your own colour if you prefer it:

\documentclass{book}
\usepackage{mathtools}
\usepackage{newtxtext}
\usepackage{newtxmath}
\PassOptionsToPackage{svgnames, dvipsnames, table, x11names}{xcolor}
\usepackage{tikz}
\usepackage{classico}
\usepackage{adforn}
\definecolor{ocre}{RGB}{243,102,25}
%----------------------------------------------------------------------------------
% Change number equations with sections
%----------------------------------------------------------------------------------
\counterwithin{equation}{section}%
\renewcommand{\theequation}{\thesection-\arabic{equation}}
\newtagform{arrowtip}[\bfseries\sffamily]{}{\,\textcolor{IndianRed3}{\Large\adforn{42}}}
\usetagform{arrowtip}

\begin{document}

\setcounter{chapter}{2}
\setcounter{section}{5}
\begin{equation}
\oint \mathbf{E} \cdot d \mathbf{a} = \int \rho \, dv
\end{equation}
\begin{equation}
\oint \mathbf{B} \cdot d \mathbf{a} = \mathbf{0}
\end{equation}

\end{document}