- 更新 Trivial group is a subgroup of all groups 的說明
- Multiplication inverse of two sided unit is unique
- If an element is a (left) unit then its (right) action is injective
- An element is a (left) unit if and only if (left) action has a section
- (Left) Unit
- (Left) Action
- An element is not a (left) zero divisor if and only if (left) action is injective
- Non zero-divisor
- 負負得正
- 開始寫 Ring 理論