Product Inverse Operation is Self-Inverse

Theorem

Let $\struct {G, \circ}$ be a group whose identity is $e$.

Let $\oplus: G \times G \to G$ be the product inverse of $\circ$ on $G$.

Then $\oplus$ is self inverse in the sense that:

$\forall x \in G: x \oplus x = e$

$\ds \forall x \in G: \, $

$\ds x \oplus x$

$=$

$\ds x \circ x^{-1}$

Definition of Product Inverse Operation

$\ds $

$=$

$\ds e$

Group Axiom $\text G 3$: Existence of Inverse Element

$\blacksquare$

1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text I$: Algebraic Structures: $\S 7$: Semigroups and Groups: Exercise $7.7 \ \text {(a)}: 1^\circ$