Contents

foundations

# Contents

## Idea

The Hartogs number of a cardinal number $\kappa$ is the number of ways to well-order a set of cardinality at most $\kappa$.1

Assuming the axiom of choice, it is the smallest ordinal number whose cardinality is greater than $\kappa$ and therefore the successor of $\kappa$ as a cardinal number. But even without the axiom of choice, it makes sense and is often an effective substitute for such a successor.

## Definition

We will define the Hartogs number as a functorial operation from sets to well-ordered sets. The operation on numbers is just a round-about way of talking about the same thing.

So let $S$ be a set. Without the axiom of choice (or more precisely, the well-ordering theorem), it may not be possible to well-order $S$ itself, but we can certainly well-order some subsets of $S$. On the other hand, if we can well-order $S$ (or a subset), then there may be many different ways to do so, even nonisomorphic ways. So to begin with, let us form the collection of all well-ordered subsets of $S$, that is the subset of

$\coprod_{A: \mathcal{P}S} \mathcal{P}(A \times A) ,$

where $\coprod$ indicates disjoint union and $\mathcal{P}$ indicates power set, consisting of those pairs $(A,R)$ such that $R$ is a well-ordering. Then form a quotient set by identifying all well-ordered subsets that are isomorphic as well-ordered sets. This gives a set of well-order types, or ordinal numbers, which can itself be well-ordered by the general theory of ordinal numbers.

The Hartogs number of $S$ is this well-ordered set, the set of all order types of well-ordered subsets of $S$. It is denoted $\aleph(S)$. If $\kappa$ is the cardinality of $S$, then let $\kappa^+$ be the cardinality or ordinal rank (as desired) of the Hartogs number of $S$; this is called the Hartogs number of $\kappa$.

There are other ways to encode $\aleph(S)$. A subset $A \subseteq S$ with a well-ordering can be specified through the collection of its initial segments or down-sets, i.e., a suitable family of subsets of $S$ named by an element of $P P(S)$. Thus the set of well-orderings of subsets of $S$ can be defined as a suitable subset of $P P(S)$. Similarly, an equivalence class of such structures corresponding to a given order type is also definable as a subset of $P P(S)$, i.e., is named by a suitable element of $P P P(S)$, so $\aleph(S)$ which is the set of such equivalence classes is definable as a subset of $P P P(S)$.

All of the defining formulas in this description involve only bounded quantification, i.e., can be expressed in the language of BZC or in ETCS. The proof that $\aleph(S)$ is well-ordered (by existence of $P$-coalgebra morphisms between representatives of order types) can be enacted in these theories without using the axiom of choice, hence the basic theory of $\aleph(S)$ can be carried out in BZ (bounded Zermelo set theory, without choice).

## Properties

###### Theorem

There is no injection $\aleph(S) \to S$.

This theorem is to Cantor's theorem as Burali-Forti's paradox is to Russell's paradox. This observation alone hints how the proof goes:

###### Proof

As observed before, $\aleph(S)$ is well-ordered. Suppose there were an injection $i: \aleph(S) \to S$; then $i$ gives a bijection of $\aleph(S)$ onto its image $I$. By transport of structure, $I$ uniquely admits a well-ordering for which $i$ is an isomorphism of well-ordered sets. Using this well-ordering, there is an initial segment inclusion

$j: I \hookrightarrow \aleph(S)$

which takes $x \in I$ to (the order type of) $\{y \in I: y \lt x\}$. Since there can be at most one initial segment inclusion between well-orders (cf. simulation), $j$ must coincide with the isomorphism $i^{-1}$, which is onto. This means $I$ itself, as an order type, must be of the form $I = \{y \in I: y \lt x\}$ for some $x \in I$, which in turn implies $x \lt x$, a contradiction.

According to this theorem, using the usual ordering of cardinal numbers, $\kappa^+ \nleq \kappa$. So if this $\leq$ is a total order (a statement equivalent to the axiom of choice), we can say that $\kappa^+ \gt \kappa$.

Even without choice, however, we can say this: If $\alpha$ is an ordinal number such that $|\alpha| \nleq \kappa$, then $\kappa^+ \leq \alpha$. (Notice that we've shifted our thinking of the Hartogs number from a cardinal to an ordinal.) That is, $\kappa^+$ is the smallest ordinal number whose cardinal number is not at most $\kappa$. This doesn't use any form of choice except for excluded middle; we only need choice to conclude that $|\kappa^+| \gt \kappa$.

The axiom of choice also implies the well-ordering theorem, that any set can be well-ordered. Thus with choice, $\kappa^+$ is (now as a cardinal again) the smallest cardinal number greater than $\kappa$; this explains the notation $\kappa^+$.

### GCH implies AC

In ZF (without C, the axiom of choice), a set $X$ can be well-ordered if there is an injection $i: X \to \aleph(X)$, for any subset of a well-ordered set inherits a well-order. As an amusing illustration of this principle, we will prove an old result due to Sierpiński, following Gillman:

###### Theorem

If the generalized continuum hypothesis holds, then the axiom of choice holds, i.e., ZF + GCH implies AC.

(Recall the GCH as formulated in ZF: for infinite sets $Y, Z$, if $|Y| \leq |Z| \leq |P(Y)|$, then $|Y| = |Z|$ or $|Z| = |P(Y)|$, letting $|Y|$ denote the cardinality as usual and where $|Y| \leq |Z|$ means there is a monomorphism $Y \to Z$.)

###### Proof

In fact we will show that any set $Y$ can be well-ordered, which implies AC (see the article Zorn's lemma). We start with a technical trick: any $Y$ can be embedded in a set $X$ with the property $|X| = |X + X|$ (consider for example $X = P(\mathbb{N} + Y)$), and then it suffices to well-order such $X$, since we can then pull back the well-ordering along the embedding to well-order $Y$. Iterated power sets of $X$ also have that property.

Now, the Hartogs number $\aleph(X)$ can be constructed explicitly in ZF as a subset of the triple power set $P^3(X) = P P P(X)$. We will show for $n \in \{1, 2, 3\}$ that under GCH, the hypothesis $|\aleph(X)| \leq |P^n(X)|$, true for $n = 3$, leads to one of two conclusions:

1. $|X| \leq |\aleph(X)|$ (whence $X$ can be well-ordered), or

2. $|\aleph(X)| \leq |P^{n-1}(X)|$.

If we land in case 2, then we loop around and feed that conclusion back into the hypothesis and iterate. Iterating this, we cannot descend through case 2 all the way down to $|\aleph(X)| \leq |P^0(X)| = |X|$, so after a few iterations we wind up in case 1 anyway: $X$ can be well-ordered.

To show this, we need a spot of cardinal arithmetic in ZF. First, we already observed that any $P = P^n(X)$ for $n \geq 0$ has the property $|P| = |2P|$. Second, we claim

• If $P, A$ are sets such that $|P| = |2P|$ and $|A+P| = |2^P|$, then $|2^P| \leq |A|$

whose ZF-proof we give in a moment. Granting the claim, let us continue. We have

$|P^{n-1}(X)| \leq |\aleph(X)| + |P^{n-1}(X)| \stackrel{hypoth}{\leq} |P^n(X)| + |P^{n-1}(X)| \leq 2|P^n(X)| = |P^n(X)|$

or just

$|P^{n-1}(X)| \leq |\aleph(X)| + |P^{n-1}(X)| \leq |P^n(X)|.$

By GCH, one of those two inequalities is an equality. If the first inequality is an equality, then we conclude $|\aleph(X)| \leq |P^{n-1}(X)|$, which lands us in case 2. If the second inequality is an equality, then the claim applies and we conclude $|P^n(X)| \leq |\aleph(X)|$, in which case $|X| \leq |P^n(X)| \leq |\aleph(X)|$ and we land in case 1.

So we are done except for the claim. Given bijections $A + P \cong 2^P$, $P + P \cong P$, let $f$ be the evident composite

$P \stackrel{incl}{\to} A + P \cong 2^P \cong 2^{P + P} \cong 2^P \times 2^P \stackrel{\pi_1}{\to} 2^P$

where $\pi_1$ denotes first projection. Let $C$ be an element not in the image of $f$, e.g., $C = \{x \in P: x \notin f(x)\}$ (Cantor's theorem). It follows that under the bijection $A + P \cong 2^P \times 2^P$, those elements of $A + P$ that map to elements of the fiber $\pi_1^{-1}(\{C\})$ can only belong to the summand $A$. Thus it is a subset of $A$ that maps bijectively onto $\pi_1^{-1}(\{C\}) \cong 2^P$, and this proves $|2^P| \leq |A|$.

### Equivalent forms of AC

Methods somewhat similar to those used to prove GCH implies AC can be used to establish equivalent forms of AC.

###### Theorem

(Tarski) Over ZF (or ETCS), the axiom of choice holds iff every infinite set $Y$ can be put into bijection with its square: $Y^2 \cong Y$.

###### Proof

The more interesting direction is the “if”, so we assume every infinite set can be put into bijection with its square.

It suffices to prove that every $X$ can be well-ordered. WLOG we prove this for (Dedekind) infinite $X$, since every $X$ embeds in a Dedekind infinite set (such as $X + \mathbb{N}$), and we can pull back a well-order on the latter along such an embedding to one on the former.

For infinite $X$, let $Y$ be the disjoint union $X + \aleph(X)$, and let $\phi: Y \times Y \to Y$ be a bijection. We claim that for all $x \in X$, there exist $\alpha, \beta \in \aleph(X)$ such that $\phi(x, \alpha) = \beta$. If that were not true, then there would exist $x \in X$ such that for all $\alpha \in \aleph(X)$ we have $\phi(x, \alpha) \notin \aleph(X)$, or in other words for all $\alpha \in \aleph(X)$ we have $\phi(x, \alpha) \in X$, so that $\phi(x, -)$ defines an injection $\aleph(X) \rightarrowtail X$. This is impossible.

By the claim, for each $x \in X$ there is a least pair $(\alpha_x, \beta_x) \in \aleph(X)^2$, considered in lexicographic order, such that $\phi(x, \alpha_x) = \beta_x$. This assignment $x \mapsto (\alpha_x, \beta_x)$ defines an embedding $X \to \aleph(X)^2$ into a well-ordered set, so that $X$ inherits a well-order by restriction along the embedding, and we are done.

A very similar method establishes the following claim.

###### Proposition

Over ZF, AC is equivalent to the statement that every inhabited set admits a group structure.

###### Proof

Every inhabited finite set admits a cyclic group structure, and under AC any infinite set can be put in bijection with the collection of its finite subsets, which forms a group under symmetric difference.

Conversely, suppose $Y = X + \aleph(X)$ admits a group structure. Then for every $x \in X$, there exist $\alpha, \beta \in \aleph(X)$ such that $x\alpha = \beta$. For if that were not true, then there exists $x \in X$ such that for all $\alpha \in \aleph(X)$, we have $x \alpha \notin \aleph(X)$, i.e., for all $\alpha \in \aleph(X)$ we have $x\alpha \in X$, which implies left multiplication by $x$ defines an injection $\aleph(X) \rightarrowtail X$, contradiction.

So for each $x \in X$, there exists a least $(\alpha_x, \beta_x) \in \aleph(X)^2$ in lexicographic order such that $x \alpha_x = \beta_x$. This again defines an embedding $x \mapsto (\alpha_x, \beta_x)$ into a well-ordered set $\aleph(X)^2$, and we are done.

## Examples

For $n$ a natural number regarded as the cardinal number of a finite set, $n^+$ is the usual successor $n + 1$. This result uses excluded middle; else we get the plump successor of $n$, which may be rather larger.

For $\aleph_0$ the cardinality of the set of all natural numbers, the Hartogs number $\aleph_0^+ = \omega_1$ is the smallest uncountable ordinal. Assuming the axiom of choice (countable choice and excluded middle are enough), we have $\aleph_0^+ = \aleph_1$ as a cardinal.

In general, we get a sequence $\omega_\alpha$ of infinite cardinalities of well-orderable sets; assuming excluded middle, every infinite well-orderable cardinality shows up in this sequence. Assuming the axiom of choice, every infinite cardinal shows up, and we have $|\omega_\alpha| = \aleph_\alpha$. (Actually, there's no real need to begin with infinite cardinals; if we started with $\omega_0 = 0$ instead of $\omega_0 = \mathbf{N}$ and $\aleph_0 = 0$ instead of $\aleph_0 = |\mathbf{N}|$, then absolutely every cardinality or well-orderable cardinality would appear.)

• Friedrich Hartogs, Über das Problem der Wohlordnung , Math. Ann. 76 no.4 (1915) pp.438-443. (gdz)

• Radu Diaconescu, On Comparability in a Topos , Proc. AMS 98 no.3 (1986) pp.389-393. (pdf)

• Leonard Gillman, Two Classical Surprises Concerning the Axiom of Choice and the Continuum Hypothesis, Amer. Math. Monthly 109 (June-July 2002), 544-553. (pdf)

The last section of Gillman’s paper is based on Sierpiński’s 1947 proof which may be found here.

1. The spelling Hartogs’ number, or even Hartog’s number, is also in use. The latter based presumably on a misapprehension since the concept was originally proposed by the German mathematician Friedrich Hartogs (1874-1943) in 1915.