Relation Compatible with Group Operation is Strongly Compatible/Corollary
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Theorem
Let $\struct {G, \circ}$ be a group with identity $e$.
Let $\RR$ be a relation compatible with $\circ$.
The following equivalences hold:
- $\forall x, y \in G:$
- $(1): \quad x \mathrel \RR y \iff e \mathrel \RR y \circ x^{-1}$
- $(2): \quad x \mathrel \RR y \iff e \mathrel \RR x^{-1} \circ y$
- $(3): \quad x \mathrel \RR y \iff x \circ y^{-1} \mathrel \RR e$
- $(4): \quad x \mathrel \RR y \iff y^{-1} \circ x \mathrel \RR e$
Proof
Applying Relation Compatible with Group Operation is Strongly Compatible to $x$, $y$, and $x^{-1}$ we obtain:
- $x \mathrel \RR y \iff x \circ x^{-1} \mathrel \RR y \circ x^{-1}$
- $x \mathrel \RR y \iff x^{-1} \circ x \mathrel \RR x^{-1} \circ y$
Applying Relation Compatible with Group Operation is Strongly Compatible to $x$, $y$, and $y^{-1}$, on the other hand, yields:
- $x \mathrel \RR y \iff x \circ y^{-1} \mathrel \RR y \circ y^{-1}$
- $x \mathrel \RR y \iff y^{-1} \circ x \mathrel \RR y^{-1} \circ y$
By the definition of inverse element:
- $x \circ x^{-1} = x^{-1} \circ x = y \circ y^{-1} = y^{-1} \circ y = e$
Making these substitutions proves the theorem.
$\blacksquare$