Well-Ordering Theorem

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Theorem

Every set is well-orderable.

Proof

Let $S$ be a set.

Let $\mathcal P \left({S}\right)$ be the power set of $S$.

By the Axiom of Choice, there is a choice function $c$ defined on $\mathcal P \left({S}\right) \setminus \left\{{\varnothing}\right\}$.

We will use $c$ and the Principle of Transfinite Induction to define a bijection between $S$ and some ordinal.

Intuitively, we start by pairing $c \left({S}\right)$ with $0$, and then keep extending the bijection by pairing $c \left({S \setminus X}\right)$ with $\alpha$, where $X$ is the set of elements already dealt with.

Base case
$\alpha = 0$

Let $s_0 = c \left({S}\right)$.

Inductive step
Suppose $s_\beta$ has been defined for all $\beta < \alpha$.

If $S - \left\{{s_\beta: \beta < \alpha}\right\}$ is empty, we stop.

Otherwise, define:

$s_\alpha := c \left({S - \left\{{s_\beta: \beta < \alpha}\right\} }\right)$.

The process eventually stops, else we have defined bijections between subsets of $S$ and arbitrarily large ordinals.

Now, we can impose a well-ordering on $S$ by embedding it via $s_\alpha \to \alpha$ into the ordinal $\beta = \displaystyle{\bigcup_{s_\alpha \in S} \alpha}$ and using the well-ordering of $\beta$.

$\blacksquare$

Also known as

This result is also known as Zermelo's Theorem, for Ernst Friedrich Ferdinand Zermelo.

Under this name it can often be seen worded:

Every set of cardinals is well-ordered with respect to $\le$.

This is called by some authors the Trichotomy Problem.

It is also referred to as the well-ordering principle, but this causes confusion with the result that states that the natural numbers are well-ordered.

Axiom of Choice

This theorem depends on the Axiom of Choice.

Because of some of its bewilderingly paradoxical implications, the Axiom of Choice is considered in some mathematical circles to be controversial.

Most mathematicians are convinced of its truth and insist that it should nowadays be generally accepted.

However, others consider its implications so counter-intuitive and nonsensical that they adopt the philosophical position that it cannot be true.