\documentclass[a4paper, 11pt,
  % use font = times, % garamond, newcm, lmodern, ...
  % use osf,
  use style = plain,
  % use indent = false,
  title in boldface,
  % title in scshape,
  % title in sffamily,
  theorem in new line,
  emphasize theorems,
  theorem numbering = section,
  % number theorems separately,
  simple name,  % for clever referencing
  name in link, % for clever referencing
]{einfart}
\usepackage{ProjLib}
\usepackage{blindtext}

%% \UseLanguage{French}

\numberwithin{equation}{section}
\SwitchTheoremNumbering{example,examples,claim}

\begin{document}

\title{Title of the article (demo of the style \textquote{plain})}
\author{Author One}
\address{Address of Author One}
\email{\href{mailto:author.one@email.com}{author.one@email.com}}
%% \thanks{The first author is indebted to...}
\author{Author Two}
\address{Address of Author Two}
\email{\href{mailto:author.two@email.com}{author.two@email.com}}
%% \thanks{The second author would like to thank the support of...}
\subjclass{XXXXX}
%% \keywords{AAA, BBB, CCC}
\date{\today[only-year-month]}
%% \date{\TheDate{2024-01}}

\maketitle

\begin{abstract}
    Some abstract. \blindtext
\end{abstract}

\begin{keyword}
    AAA, BBB, CCC
\end{keyword}

\tableofcontents

\section{Section title}

\subsection{Subsection title}

Some introductory text.

\begin{definition}[something]
    Define something.
\end{definition}

\begin{theorem}[some result]\label{thm:result}\index{Theorem on some result}
    The statement.
\end{theorem}

\begin{emphasis}
    Clever reference: \cref{thm:result} (only name: \namecref{thm:result}, only number: \labelcref{thm:result}).
\end{emphasis}

An equation:\index{Euler's equation}
\begin{equation}\label{eq:Euler}
    \mathrm{e}^{\mathrm{i}\pi} + 1 = 0.
\end{equation}

\begin{emphasis}
    Reference of equation: \eqref{eq:Euler} or \cref{eq:Euler}.
\end{emphasis}

\SetTheorem{corollary}{shared counter = subsubsection}
\begin{corollary}[of \cref{thm:result}]\label{cor:result}
    Some corollary.
\end{corollary}
\SetTheorem{corollary}{shared counter = subsection}
\begin{proof}
    Some lines of proof.
\end{proof}

\begin{examples}
    \begin{enumerate}
        \item First example.
        \item Second example.
        \item Third example.
    \end{enumerate}
\end{examples}

\begin{example*}[Important example]
    The numbering is switched in the preamble, thus \verb|example*| is the numbered version while \verb|example| is the unnumbered version.
\end{example*}

\begin{proof}[Proof of \cref{thm:result}]
    Some lines of proof.
\end{proof}

\blinddocument

\printindex

\end{document}