Algebra
《Algebra》 · 第 42 页 · Chapter II，Section 1.2，Example 1.3
Since we explicitly require $G$ to be nonempty, the most economical way to concoct a group is by letting $G=\{e\}$ be a singleton. There is only one function $G\times G\to G$ in this case, so there is only one possible binary operation on $G$, defined by $e\mathbin{\bullet}e:=e$. The three axioms trivially hold for this example, so $\{e\}$ is equipped with a unique group structure. This is usually called the trivial group.