Solitons: Differential equations, symmetries, and infinite-dimensional algebras
《Solitons: Differential equations, symmetries, and infinite-dimensional algebras》 · 第 295 页 · Definition 11.10
A $G$-sheaf $T$ is called a $G$-torsor (for the Zariski topology) if it satisfies the following two properties. (a) The group $G(U)$ acts simply transitively on $T(U)$ for every open subset $U\subseteq X$. (b) There exists an open covering $(U_i)_i$ of $X$ such that $T(U_i)\neq\varnothing$ for all $i$.