Axiom of Choice Implies Zorn's Lemma

Theorem

Acceptance of the Axiom of Choice implies the truth of Zorn's Lemma.

Statement of Axiom of Choice

For every set of non-empty sets, it is possible to provide a mechanism for choosing one element of each element of the set.

$\displaystyle \forall s: \paren {\O \notin s \implies \exists \paren {f: s \to \bigcup s}: \forall t \in s: \map f t \in t}$

That is, one can always create a choice function for selecting one element from each element of the set.

Statement of Zorn's Lemma

Let $\struct {X, \preceq}, X \ne \O$ be a non-empty ordered set such that every non-empty chain in $X$ has an upper bound in $X$.

Then $X$ has at least one maximal element.

Proof 1

For each $x \in X$, consider the lower closure $x^\preceq$:

$x^\preceq = \set {y \in X: y \preceq x}$

Let $\mathbb S \subseteq \powerset X$ be the image of $\cdot^\preceq$ considered as a mapping from $X$ to $\powerset X$, where $\powerset X$ is the power set of $X$.

From Ordering is Equivalent to Subset Relation:

$\forall x, y \in X: x^\preceq \subseteq y^\preceq \iff x \preceq y$

Thus the task of finding a maximal element of $X$ is equivalent to finding a maximal set in $\mathbb S$.

Thus the statement of the result is equivalent to a statement about chains in $\mathbb S$:

Let $\mathbb S$ be a non-empty subset of $\powerset X, X \ne \O$ such that every non-empty chain in $\mathbb S$, ordered by $\subseteq$, has an upper bound in $\mathbb S$.

Then $\mathbb S$ has at least one maximal set.

Let $\mathbb X$ be the set of all chains in $\struct {X, \preceq}$.

Every element of $X$ is included in $\map {s^\preceq} x$ for some $x \in X$.

$\mathbb X$ is a non-empty set of sets which are ordered (perhaps partially) by subset relation.

If $\CC$ is a chain in $\mathbb X$, then:

$\displaystyle \bigcup_{A \mathop \in \CC} A \in \mathbb X$

Since each set in $\mathbb X$ is dominated by some set in $\mathbb S$, going from $\mathbb S$ to $\mathbb X$ can not introduce any new maximal elements.

The main advantage of using $\mathbb X$ is that the chain hypothesis is in a slightly more specific form.

Instead of saying that each chain in $\CC$ has an upper bound in $\mathbb S$, we can explicitly state that the union of the sets of $\CC$ is an element of $\mathbb X$.

This union of the sets of $\CC$ is clearly an upper bound of $\CC$.

Another advantage of $\mathbb X$ is that, from Subset of Toset is Toset, it contains all the subsets of each of its sets.

Thus we can enlarge non-maximal sets in $\mathbb X$ one element at a time.

So, from now on, we need consider only this non-empty collection $\mathbb X$ of subsets of a non-empty set $X$.

$\mathbb X$ is subject to two conditions:

$(1): \quad$ Every subset of each set in $\mathbb X$ is in $\mathbb X$.

$(2): \quad$ The union of each chain of sets in $\mathbb X$ is in $\mathbb X$.

It follows from $(1)$ that $\O \in \mathbb X$.

We need to show that there exists a maximal set in $\mathbb X$.

Let $f$ be a choice function for $X$:

$\forall A \in \powerset X \setminus \O: \map f A \in A$

For each $A \in \mathbb X$, let $\hat A$ be defined as:

$\hat A := \set {x \in X: A \cup \set x \in \mathbb X}$

That is, $\hat A$ consists of all the elements of $X$ which, when added to $A$, make a set which is also in $\mathbb X$.

From its definition:

$\ds \hat A = \bigcup_{x \mathop \in \hat A} \paren {A \cup \set x}$

where each of $A \cup \set x$ are chains in $X$ and so elements of $\mathbb X$.

Suppose there exists a maximal set $M$ in $\mathbb X$.

Then, by definition of maximal, there are no elements in $x \in X \setminus M$ which can be added to $M$ to make $M \cup \set x$ another element of $\mathbb X$.

Thus it follows that $\hat M = M$.

From Set Difference with Self is Empty Set it then follows that if $A$ is maximal, then $\hat A \setminus A = \O$.

The mapping $g: \mathbb X \to \mathbb X$ can now be defined as:

$\forall A \in \mathbb X: \map g A = \begin{cases} A \cup \set {\map f {\hat A \setminus A} } & : \hat A \setminus A \ne \O \\ A & : \text{otherwise} \end{cases}$

Thus what we now have to prove is that:

$\exists A \in \mathbb X: \map g A = A$

Note that from the definition of $g$:

$\forall A \in \mathbb X: A \subseteq \map g A$

Also note that $\map f {\hat A \setminus A}$ is a single element of $\hat A \setminus A$.

Thus we obtain the crucial fact that $\map g A$ contains at most one more element than $A$.

We (temporarily) define a tower as being a subset $\TT$ of $\mathbb X$ such that:

$(1): \quad \O \in \TT$

$(2): \quad A \in \TT \implies \map g A \in \TT$

$(3): \quad$ If $\CC$ is a chain in $\TT$, then $\ds \bigcup_{A \mathop \in \CC} A \in \TT$

There is of course at least one tower in $\mathbb X$, as $\mathbb X$ itself is one.

It follows from its definition that the intersection of a collection of towers is itself a tower.

It follows in particular that if $\TT_0$ is the intersection of all towers in $\mathbb X$, then $\TT_0$ is the smallest tower in $\mathbb X$.

Next we demonstrate that $\TT_0$ is a chain.

We (temporarily) define a set $C \in \TT_0$ as comparable if it is comparable with every element of $\TT_0$.

That is, if $A \in \TT_0$ then $C \subseteq A$ or $A \subseteq C$.

To say that $\TT_0$ is a chain means that all sets of $\TT_0$ are comparable.

There is at least one comparable set in $\T T_0$, as $\O$ is one of them.

So, suppose $C \in \TT_0$ is comparable.

Let $A \in \TT_0$ such that $A \subsetneq C$.

Consider $\map g A$.

Because $C$ is comparable, either $C \subsetneq \map g A$ or $\map g A \subseteq C$.

In the former case $A$ is a proper subset of a proper subset of $\map g A$.

This contradicts the fact that $\map g A \setminus A$ can be no more than a singleton.

Thus if such an $A$ exists, we have that:

$(A): \quad \map g A \subseteq C$

Now let $\UU$ be the set defined as:

$\UU := \set {A \in \TT_0: A \subseteq C \lor \map g C \subseteq A}$

Let $\UU'$ be the set defined as:

$\UU' := \set {A \in \TT_0: A \subseteq \map g C \lor \map g C \subseteq A}$

That is, $\UU'$ is the set of all sets in $\TT_0$ which are comparable with $\map g C$.

If $A \in \UU$, then as $C \subseteq \map g C$, either $A \subseteq \map g C \lor \map g C \subseteq A$

So $\UU \subseteq \UU'$.

The aim now is to demonstrate that $\UU$ is a tower.

From Empty Set is Subset of All Sets, $\O \subseteq C$.

Hence condition $(1)$ is satisfied.

Now let $A \in \UU$.

As $C$ is comparable, there are three possibilities:

$(1'): \quad A \subsetneq C$

Then from $(A)$ above, $\map g A \subseteq C$.

Therefore $\map g A \in \UU$.

$(2'): \quad A = C$

Then $\map g A = \map g C$ and so $\map g C \subseteq \map g A$.

Therefore $\map g A \in \UU$.

$(3'): \quad \map g C \subseteq A$

Then $\map g C \subseteq \map g A$

Therefore $\map g A \in \UU$.

Hence condition $(2)$ is satisfied.

From the definition of $\UU$, it follows immediately that the union of a chain in $\UU$ is also in $\UU$.

Hence condition $(3)$ is satisfied.

The conclusion is that $\UU$ is a tower such that $\UU \subseteq \TT_0$.

But as $\TT_0$ is the smallest tower, $\TT_0 \subseteq \UU$.

It follows that $\UU = \TT_0$.

Consider some comparable set $C$, then.

From that $C$ we can form $\UU$, as above.

But as $\UU = \TT_0$:

$A \in \TT_0 \implies \paren {A \subseteq C \implies A \subseteq \map g C} \lor \map g C \subseteq A$

and so $g \left({C}\right)$ is also comparable.

We now know that:

$\O$ is comparable

the mapping $g$ maps comparable sets to comparable sets.

Since the union of a chain of comparable sets is itself comparable, it follows that the comparable sets all form a tower $\TT_C$.

But by the nature of $\TT_0$ it follows that $\TT_0 \subseteq \TT_C$.

So the elements of $\TT_0$ must all be comparable.

Since $\TT_0$ is a chain, the union $M$ of all the sets in $\TT_0$ is itself a set in $\TT_0$.

Since the union includes all the sets of $\TT_0$, it follows that $\map g M \subseteq M$.

Since it is always the case that $M \subseteq \map g M$, it follows that $M = \map g M$.

The result follows.

$\blacksquare$

Proof 2

Aiming for a contradiction, suppose that for each $x \in X$ there is a $y \in X$ such that $x \prec y$.

By the Axiom of Choice, there is a mapping $f: X \to X$ such that:

$\forall x \in X: x \prec \map f x$

Let $\CC$ be the set of all chains in $X$.

By the premise, each element of $\CC$ has an upper bound in $X$.

Thus by the Axiom of Choice, there is a mapping $g: \CC \to X$ such that for each $C \in \CC$, $\map g C$ is an upper bound of $C$.

Let $p$ be an arbitrary element of $X$.

Define a mapping $h: \operatorname {Ord} \to X$ by transfinite recursion thus:

\(\ds \map h 0\)

\(=\)

\(\ds p\)

\(\ds \map h {\alpha^+}\)

\(=\)

\(\ds \map f {\map h \alpha}\)

\(\ds \map h \lambda\)

\(=\)

\(\ds \map f {\map g {f \sqbrk \lambda} }\)

if $\lambda$ is a limit ordinal

Then $h$ is strictly increasing, and thus injective.

Let $h'$ be the restriction of $h$ to $\On \times \map h \On$.

Then ${h'}^{-1}$ is a surjection from $\map h \On \subseteq X$ onto $\On$.

By the Axiom of Replacement, $\On$ is a set.

By Burali-Forti Paradox, this is a contradiction.

Thus we conclude that some element of $X$ has no strict successor, and is thus maximal.

$\blacksquare$

Also see

• Hausdorff Maximal Principle
• Kuratowski's Lemma