Solitons: Differential equations, symmetries, and infinite-dimensional algebras
《Solitons: Differential equations, symmetries, and infinite-dimensional algebras》 · 第 231 页 · Definition 9.1(1)
Let $u:X\to S$ be an $S$-object. The morphism $\Delta_{X/S}:=\Delta_u:=(\operatorname{id}_X,\operatorname{id}_X)_S:X\to X\times_SX$ is called the diagonal (morphism) of $X$ over $S$.