Definition:Minimal/Element
Definition
Let $\struct {S, \RR}$ be a relational structure.
Let $T \subseteq S$ be a subset of $S$.
Definition 1
An element $x \in T$ is a minimal element (under $\RR$) of $T$ if and only if:
- $\forall y \in T: y \mathrel \RR x \implies x = y$
Definition 2
An element $x \in T$ is a minimal element (under $\RR$) of $T$ if and only if:
- $\neg \exists y \in T: y \mathrel {\RR^\ne} x$
where $y \mathrel {\RR^\ne} x$ denotes that $y \mathrel \RR x$ but $y \ne x$.
Comparison with Smallest Element
Compare the definition of a minimal element with that of a smallest element.
Consider the ordered set $\struct {S, \preceq}$ such that $T \subseteq S$.
An element $x \in T$ is the smallest element of $T$ if and only if:
- $\forall y \in T: x \preceq y$
That is, $x$ is comparable with, and precedes, or is equal to, every $y \in T$.
An element $x \in T$ is a minimal element of $T$ if and only if:
- $y \preceq x \implies x = y$
That is, $x$ precedes, or is equal to, every $y \in T$ which is comparable with $x$.
If all elements are comparable with $x$, then such a minimal element is indeed the smallest element.
Note that when an ordered set is in fact a totally ordered set, the terms minimal element and smallest element are equivalent.
Also defined as
Most treatments of the concept of a minimal element restrict the definition of the relation $\RR$ to the requirement that it be an ordering.
However, this is not strictly required, and this more general definition as used on $\mathsf{Pr} \infty \mathsf{fWiki}$ is of far more use.
Also known as
Some sources refer to a minimal element as an atom.
However, the latter term has a meaning in several different contexts, so will not be used like this on $\mathsf{Pr} \infty \mathsf{fWiki}$.
Examples
Finite Subsets of Natural Numbers
Let $\FF$ denote the set of finite subsets of the natural numbers $\N$.
Consider the ordered set $\struct {\FF, \subseteq}$.
There is one minimal element of $\struct {\FF, \subseteq}$, and that is the empty set $\O$.
Finite Subsets of Natural Numbers less Empty Set
Let $\FF$ denote the set of finite subsets of the natural numbers $\N$.
Let $\GG$ denote the set $\FF \setminus \set \O$, that is, $\FF$ with the empty set excluded.
Consider the ordered set $\struct {\GG, \subseteq}$.
The minimal elements of $\struct {\GG, \subseteq}$ are the sets of the form $\set n$, for $n \in \N$.
Also see
- Results about minimal elements can be found here.