Equivalence of Definitions of Minimal Element

Theorem
Let $\struct {S, \RR}$ be a relational structure.

Let $T \subseteq S$ be a subset of $S$.

Definition 1 implies Definition 2
Let $x$ be an minimal element by definition 1.

That is:
 * $(1): \quad \forall y \in T: y \mathrel \RR x \implies x = y$


 * $\exists y \in T: y \mathrel {\RR^\ne} x$
 * $\exists y \in T: y \mathrel {\RR^\ne} x$

Then by definition:
 * $y \mathrel \RR x \land x \ne y$

which contradicts $(1)$.

Thus by Proof by Contradiction:
 * $\nexists y \in T: y \mathrel {\RR^\ne} x$

That is $x$ is a minimal element by definition 2.

Definition 2 implies Definition 1
Let $x$ be a minimal element by definition 2.

That is:
 * $(2): \quad \nexists y \in T: y \mathrel {\RR^\ne} x$


 * $\exists y \in T: y \mathrel \RR x: x \ne y$
 * $\exists y \in T: y \mathrel \RR x: x \ne y$

That is:
 * $\exists y \in T: y \mathrel {\RR^\ne} x$

which contradicts $(2)$.

Thus:
 * $\forall y \in T: y \mathrel \RR x \implies x = y$

Thus $x$ is a minimal element by definition 1.