In mathematics, the Teichmüller–Tukey lemma (sometimes named just Tukey's lemma), named after John Tukey and Oswald Teichmüller, states that every nonempty collection of finite character has a maximal element with respect to inclusion. Over Zermelo–Fraenkel set theory, the Teichmüller–Tukey lemma is equivalent to the axiom of choice, and therefore to the well-ordering theorem, Zorn's lemma, and the Hausdorff maximal principle.
A family of sets is of finite character provided it has the following properties:
- For each , every finite subset of belongs to .
- If every finite subset of a given set belongs to , then belongs to .
Statement of the Lemma
Whenever is of finite character and , there is a maximal such that .
In linear algebra, the lemma may be used to show the existence of a basis. Let V be a vector space. Consider the collection of linearly independent sets of vectors. This is a collection of finite character Thus, a maximal set exists, which must then span V and be a basis for V.
- Brillinger, David R. "John Wilder Tukey" 
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