% !TEX encoding = ITF-8 Unicode
\documentclass[a4 paper, 11pt]{book}
\usepackage{enumitem}??? 
\begin{document}??
\begin{enumerate}??[itemsep=0cm, topsep=0cm, partopsep=0cm,parsep=0cm]
\item??
a1
\item??
a2
\end{enumerate}
\end{document}