# Bases of Vector Space have Equal Cardinality

## Theorem

Let $k$ be a division ring.

Let $V$ be a vector space over $k$.

Let $X$ and $Y$ be bases of $V$.

Then $X$ and $Y$ are equinumerous.

## Proof

We will first prove that there is an injection from $X$ to $Y$.

Let $x \in X$.

By Expression of Vector as Linear Combination from Basis is Unique: General Result, there is a unique finite subset $C_x$ of $R \times Y$ such that:

$\displaystyle x = \sum_{\tuple {r, v} \mathop \in C_x} r \cdot v$ and
$\forall \tuple {r, v} \in C_x: r \ne 0_R$

Define $\Phi: X \to \powerset Y$ by:

$\map \Phi x := \Img {C_x}$

We next show that $\sequence {\map \Phi x}_{x \mathop \in X}$ satisfies the marriage condition.

That is, for every finite subset $F$ of $X$:

$\displaystyle \card F \le \card {\bigcup \map \Phi F}$

Since $X$ is a basis, it is linearly independent.

By the definition of $\Phi$:

$\displaystyle F \subseteq \operatorname {span} \bigcup \map \Phi F$.

By Finite Union of Finite Sets is Finite, $\displaystyle \bigcup \map \Phi F$ is finite.

$\displaystyle \size F \le \size {\bigcup \map \Phi F}$

By Hall's Marriage Theorem, there is an injection from $X$ into $Y$.

Precisely the same argument with $X$ and $Y$ interchanged shows that there is an injection from $Y$ into $X$ as well.

Thus by the Cantor-Bernstein-Schröder Theorem, $X$ and $Y$ are equinumerous.

$\blacksquare$

#### Boolean Prime Ideal Theorem

This proof depends on the Boolean Prime Ideal Theorem (BPI), by way of Hall's Marriage Theorem/General Set.

Although not as strong as the Axiom of Choice, the BPI is similarly independent of the Zermelo-Fraenkel axioms.

As such, mathematicians are generally convinced of its truth and believe that it should be generally accepted.

## Also known as

Some authors refer to this as the dimension theorem for vector spaces, but it should not be confused with e.g. the Rank Plus Nullity Theorem.