《Algebra》
· 第 227 页 · Chapter IV，Section 7，Definition 7.1
Let $K$ be a commutative ring with identity. A $K$-algebra (or algebra over $K$) $A$ is a ring $A$ such that: (i) $(A,+)$ is a unitary (left) $K$-module; (ii) $k(ab)=(ka)b=a(kb)$ for all $k\in K$ and $a,b\in A$.
