math

axiom of choice

The axiom of choice is a mathematical principle asserting that from any collection of non-empty sets, one can select an element from each set.
7 related articles

Description

The axiom of choice is one of the statements of set theory, postulating the existence of a choice function on any family of nonempty sets. That is, for any collection (possibly infinite) of pairwise disjoint nonempty sets, there exists a set containing exactly one element from each set in the collection. For a finite number of sets it seems trivial, but for infinite ones it gives a non-constructive result: we know that a choice is possible, but we cannot describe it. It is equivalent to Zermelo's well-ordering principle and Zorn's lemma.

History

First explicitly formulated by Ernst Zermelo in 1904 to prove that every set can be well-ordered. This claim was immediately challenged by Émile Borel, Henri Lebesgue and other leading analysts. In 1938, Kurt Gödel showed that AC is consistent with ZF, and in 1963 Paul Cohen proved that the negation of AC is also compatible with ZF. Thus, the axiom of choice is independent. Today, the overwhelming majority of mathematicians use AC, although alternative approaches (constructivism, topos theory) exist where it is avoided.

How it works

Functionally, AC allows non-constructive constructions. In proving theorems, one often says: 'For each i, choose an element x_i from the set A_i' without specifying a choice rule. This is valid if the axiom of choice is accepted. Without it, such reasoning is inadmissible. A typical example: the proof of the existence of a Hamel basis in an arbitrary vector space—we successively choose linearly independent vectors, and at infinite steps AC is required. Technically, the axiom does not indicate how to find the choice; it simply postulates its existence.

In practice

The foundation of many theorems in mathematics: without it, one cannot prove that every vector space has a basis, or that the Tychonoff product of compact spaces is compact. Used in analysis, topology, logic.

Fun fact

Using the axiom of choice, one can mathematically 'double' a ball: cut it into finitely many pieces, move them, and obtain two balls of the same radius. This Banach–Tarski paradox shows how powerful and counterintuitive the axiom is.

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