在第二章，我重新整理了Lenstra的筆記和Grothendieck的SGA I的5.4章節。基本上可以分成兩個部分： 第一個部分根據Lenstra筆記的第三章。有一些比較省略的步驟，還有一些步驟留作習題，我把這些地方補上並想辦法寫得更流暢些。第二個部分根據Grothendieck的SGA I的5.4章節，用pro-objects的技術，首次出現是在Seminaire Bourbaki一篇Grothendieck的文章裡。我從原來文章裡簡短的描述中，給了定理4.1一個詳細的證明。; In chapter 2, I reorganize some part of \cite{Le08} and \cite{SGA1}. Basically it can be divided into two parts: the first part follows \cite{Le08} Chapter 3. Some steps are written a bit roughly and some steps are exercises in original texts, I just make them more fluent, and write down the exercises; The second part follows \cite{SGA1} Section 5.4, using the technique so called pro-objects, first introduced by Grothendieck in his article in Seminaire Bourbaki \cite{Gr59}. I give a proof of \cite{SGA1} Expos\'{e} V. Theorem 4.1, following the brief sketch in the original article.
