Equivalence of Definitions of Maximal Element

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Theorem

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

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


The following definitions of the concept of Maximal Element are equivalent:

Definition 1

An element $x \in T$ is a maximal element (under $\RR$) of $T$ if and only if:

$x \mathrel \RR y \implies x = y$

Definition 2

An element $x \in T$ is a maximal element (under $\RR$) of $T$ if and only if:

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

where $x \mathrel {\RR^\ne} y$ denotes that $x \mathrel \RR y$ but $x \ne y$.


Proof

Definition 1 implies Definition 2

Let $x$ be an maximal element by definition 1.

That is:

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

Aiming for a contradiction, suppose that:

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

Then by definition:

$x \mathrel \RR y \land x \ne y$

which contradicts $(1)$.

Thus by Proof by Contradiction:

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

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

$\Box$


Definition 2 implies Definition 1

Let $x$ be a maximal element by definition 2.

That is:

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

Aiming for a contradiction, suppose that:

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

That is:

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

which contradicts $(2)$.

Thus:

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

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

$\blacksquare$


Sources

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