エタール基本群
https://gyazo.com/b8d6f02b0902fe6bf5cea797c137dec6
https://www.jstage.jst.go.jp/article/sugaku1947/50/2/50_2_113/_pdf/-char/ja
https://gyazo.com/b9793f20a4e19381a4c5de3abced53bb
https://webusers.imj-prg.fr/~leila.schneps/grothendieckcircle/EsquisseEng.pdf
Arithmetic of algebraic numbers ........................... 115
4.1 Algebraic Numbers: Their Realizations and Geometry . .