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\documentclass[a4paper]{jsarticle}

%section, subsection, subsubsection の先頭に部番号が付くようにする
\makeatletter
\@addtoreset{section}{part}\renewcommand{\thesection}{\thepart.\arabic{section}}
\@addtoreset{subsection}{section}\renewcommand{\thesubsection}{\thesection.\arabic{subsection}}
\@addtoreset{subsubsection}{subsection}\renewcommand{\thesubsubsection}{\thesubsection.\arabic{subsubsection}}
\makeatother

\begin{document}
	\title{部番号なし}
	\author{motchy}
	\maketitle
	
	\part{A}
		\section{B}
			\subsection{C}
				\label{subsec:C}
				Alpha Bravo Charlie
				
	\part{い}
		\setcounter{section}{0}
		\section{ろ}
			\subsection{は}
				\label{subsec:は}
				いろは にほへと ちりぬるを
	\part{ほげ}
		\setcounter{section}{0}
		\section{ふが}
			\subsection{ぴよ}
				\ref{subsec:C}, \ref{subsec:は} より...
\end{document}