Finite Character

That is,

1. For each ${\displaystyle A\in {\mathcal {F}}}$, every finite subset of ${\displaystyle A}$ belongs to ${\displaystyle {\mathcal {F}}}$.
2. If every finite subset of a given set ${\displaystyle A}$ belongs to ${\displaystyle {\mathcal {F}}}$, then ${\displaystyle A}$ belongs to ${\displaystyle {\mathcal {F}}}$.

A family ${\displaystyle {\mathcal {F}}}$  of sets of finite character enjoys the following properties:

1. For each ${\displaystyle A\in {\mathcal {F}}}$ , every (finite or infinite) subset of ${\displaystyle A}$  belongs to ${\displaystyle {\mathcal {F}}}$ .
2. Every nonempty family of finite character has a maximal element with respect to inclusion (Tukey's lemma): In ${\displaystyle {\mathcal {F}}}$ , partially ordered by inclusion, the union of every chain of elements of ${\displaystyle {\mathcal {F}}}$  also belongs to ${\displaystyle {\mathcal {F}}}$ , therefore, by Zorn's lemma, ${\displaystyle {\mathcal {F}}}$  contains at least one maximal element.

Let ${\displaystyle V}$  be a vector space, and let ${\displaystyle {\mathcal {F}}}$  be the family of linearly independent subsets of ${\displaystyle V}$ . Then ${\displaystyle {\mathcal {F}}}$  is a family of finite character (because a subset ${\displaystyle X\subseteq V}$  is linearly dependent if and only if ${\displaystyle X}$  has a finite subset which is linearly dependent). Therefore, in every vector space, there exists a maximal family of linearly independent elements. As a maximal family is a vector basis, every vector space has a (possibly infinite) vector basis.

• Hereditarily finite set

• Jech, Thomas J. (2008) [1973]. The Axiom of Choice. Dover Publications. ISBN 978-0-486-46624-8.
• Smullyan, Raymond M.; Fitting, Melvin (2010) [1996]. Set Theory and the Continuum Problem. Dover Publications. ISBN 978-0-486-47484-7.

This article incorporates material from finite character on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.