## 580 -II. Cardinal arithmetic

Homework problem 5. (${\sf ZF}$). Show that if $\omega\preceq{\mathcal P}(X)$ then in fact ${\mathcal P}(\omega)\preceq{\mathcal P}(X)$.

Question. If $X$ is Dedekind-finite but ${\mathcal P}(X)$ is Dedekind-infinite, does it follow that there is an infinite Dedekind-finite set $Y$ such that ${\mathcal P}(Y)\preceq X$?

From now on, until we cover determinacy, we assume the axiom of choice unless stated otherwise.

1. Preliminaries.

We use $\kappa,\lambda,\dots$ to denote cardinals (initial ordinals), usually infinite. As usual, $\kappa+\lambda$ is the cardinality of the disjoint union of two sets, one of size $\kappa$, and one of size $\lambda$, $\kappa+\lambda=|\kappa\sqcup\lambda|.$ In fact, we can take $\kappa+\lambda=|\kappa+\lambda|,$ where the latter $+$ denotes ordinal addition.

The advantage of doing this is that we can generalize it to infinite sums.

Definition. Let $\gamma$ be an ordinal and $(\alpha_i:i<\gamma)$ a sequence of ordinals. Then $\sum_{i<\gamma}\alpha_i$ is the ordinal resulting from concatenating the $\alpha_i$ in the given order; formally, we consider the disjoint union $\bigcup_{i\in\gamma}\alpha_i\times\{i\}$ and well-order it by setting $(\beta,i)<(\delta,j)$ iff $i or else $i=j$ and $\beta<\delta.$

It is straightforward to check that this is indeed a well-ordering.

Definition. Let $I$ be a set and let $(\kappa_i:i\in I)$ be an $I$-indexed family of cardinals. Then $\sum_{i\in I}\kappa_i=|\sum_{a<|I|}\kappa_{\pi(a)}|$ where $\pi : |I|\to I$ is a bijection, and the later sum is the infinitary addition of ordinals defined above.

One easily checks that this is well-defined (i.e., the same number is obtained for all bijections $\pi$), and coincides with the cardinality of the disjoint union of the $\kappa_i$.

Remark. Without choice, there is no reasonable way of defining $\sum_{i\in I}{\mathfrak m}_i$, even if we make the sum dependent on the specific index set $I$. For example, it is consistent to have an uncountable set that is the countable union of sets of size two. The issue is that even if there are bijections between sets $A_i$ and $B_i$ for each $i\in I,$ it is not necessarily the case that there is a function that to each $i$ assigns some such bijection, and it is indeed possible that there is no bijection between the resulting sets $\bigcup_{i\in I}A_i$ and $\bigcup_{i\in I}B_i.$

Homework problem 6. It is consistent with ${\sf ZF}$ that ${\mathbb R}$ is a countable union of countable sets. It is also consistent with ${\sf ZF}$ that there are infinite Dedekind-finite subsets of ${\mathbb R}$. However, show that these two statements cannot hold simultaneously.

The first result shows that cardinal addition trivializes in the presence of choice.

Fact 1. Let $I$ be a nonempty set. If the $\kappa_i$ are nonzero, and either $I$ or $\sup_{i\in I}\kappa_i$ is infinite, then $\sum_{i\in I}\kappa_i=\max\{|I|,\sup_{i\in I}\kappa_i\}.$ In particular, $\kappa+\lambda=\max\{\kappa,\lambda\}$ if at least one of $\kappa,\lambda$ is infinite.

Proof. First we argue the case for two cardinals: Let ${\mathfrak m}=\max\{\kappa,\lambda\}.$ Then ${\mathfrak m}\le\kappa+\lambda\le|\kappa\times\lambda|\le|{\mathfrak m}\times{\mathfrak m}|={\mathfrak m},$ where we use the fact that there is an injection $A\sqcup B\to A\times B$ whenever at least one of $A,B$ is infinite, and the fact that $\alpha$ and $\alpha\times\alpha$ have the same size, for any infinite ordinal $\alpha$, as shown  on lecture 3, lemma in section 6.

Remark. ${\mathfrak m}+{\mathfrak n}\le{\mathfrak m}\times{\mathfrak n}$ holds in the absence of choice if both ${\mathfrak m},{\mathfrak n}$ are at least two. It fails in the generality above, since if ${\mathfrak m}$ is infinite and Dedekind-finite, ${\mathfrak m}+1>{\mathfrak m}\times1.$

The result is now easy: Clearly, $\sum_{i\in I}\kappa_i\ge |I|$ and $\sum_{i\in I}\kappa_i\ge\kappa_j$ for any $j\in I$. Also, if ${\mathfrak m}=\sup_{i\in I}\kappa_i,$ then $\sum_{i\in I}\kappa_i\le\sum_i{\mathfrak m}=|{\mathfrak m}\times I|$. The result follows from the above and the Schröder-Bernstein theorem. ${\sf QED}$

Unlike addition, multiplication is significantly more difficult.

Definition. Let $I$ be a set and let $(\kappa_i:i\in I)$ be an $I$-indexed family of cardinals. Then $\prod_{i\in I}\kappa_i=|\prod_{i\in I}\kappa_i|$ where the latter $\prod$ denotes the product of the sets $\kappa_i$, the set of all (choice) functions $f:I\to\bigcup_i\kappa_i$ such that for all $i\in I$, $f(i)\in\kappa_i.$

Clearly, a product is zero is one of its factors is zero, and a finite product has the size of the Cartesian product of the sets involved.

One easily checks that if $A_i\sim B_i$ for all $i\in I$, then $\prod_iA_i\sim\prod_iB_i.$

Remark. Just as with addition, without choice there is no reasonable way of defining $\prod_{i\in I}{\mathfrak m}_i$, even if we make the product dependent on the specific index set $I$. For example, it is consistent to have a countable collection of sets of size two whose product is not in bijection with the product of countably many copies of $\{0,1\}.$

Definition. $\kappa^\lambda=|{}^\lambda\kappa|$, where ${}^A B$ denotes the set of functions from $A$ to $B$, so $\prod_{i\in I}\kappa=\kappa^{|I|}.$ Also, $\kappa\cdot\lambda=\kappa\times\lambda=\kappa\lambda$ denotes the product of $\kappa$ and $\lambda.$

During the proof of Fact 1 we showed:

Fact 2. $\kappa\lambda=\max\{\kappa,\lambda\}$ if at least one of $\kappa,\lambda$ is infinite, and neither is zero. $\Box$

From choice it follows that a product of nonempty sets is nonempty. In fact, it tends to be a rather large object. Before we present quantitative evidence of this statement, one more definition is in order. This notion is central to set theoretic combinatorics.

Definition. If $(X,<)$ is a linearly ordered set, a subset $Y\subseteq X$ is cofinal in $X$ iff $\forall x\in X\,\exists y\in Y\,(x\le y).$

Obviously, this notion coincides with the notion of an unbounded subset if $X$ has no largest element. If $X$ has a largest element, a subset of $X$ is cofinal iff it contains this largest element.

Definition. A map into a linear order $(X,<)$ is cofinal iff its range is cofinal. The cofinality ${\rm cf}(X)$ of $(X,<)$ is the smallest ordinal $\beta$ such that there is a cofinal map $f:\beta\to(X,<)$.

Lemma 3.

1. ${\rm cf}(0)=0$, ${\rm cf}(\alpha+1)=1$, and ${\rm cf}(\alpha)$ is an infinite cardinal for any limit ordinal $\alpha.$
2. ${\rm cf}({\rm cf}(\alpha))={\rm cf}(\alpha)$.
3. Any cofinal subset of a linearly ordered set $(X,<)$ contains a cofinal subset of size ${\rm cf}(X)$.

Proof. In item 1, only that ${\rm cf}(\alpha)$ is an infinite cardinal for $\alpha$ limit needs to be argued. But if there is a cofinal map $f:\beta\to\alpha$ and $\beta\sim\gamma,$ then there is a cofinal map $g:\gamma\to\alpha.$ It follows that ${\rm cf}(\alpha)$ is a cardinal. It is obviously infinite, since $\alpha$ is a limit ordinal and therefore no map from a finite set into $\alpha$ will be cofinal.

Item 2 is shown similarly: If $\beta<{\rm cf}(\alpha)$ and there is a cofinal map $f:\beta\to{\rm cf}(\alpha)$, then there is a cofinal map $g:\beta\to\alpha,$ contradicting the definition of ${\rm cf}(\alpha)$.

To prove item 3, let $Y$ be a cofinal subset of the linearly ordered set $X$ and let $Z$ be a cofinal subset of $Y$ of smallest cardinality. By transfinite recursion, define a strictly increasing sequence $\vec a= (a_\alpha:\alpha<\theta)$ of elements of $Z.$ The construction stops once the range of the sequence is cofinal in $Z.$ Let $\mu={\rm cf}(\theta)$, and select a cofinal subsequence of $\vec a$ of length $\mu$. Then this sequence witnesses that ${\rm cf}(X)\le\mu.$ By minimality of $|Z|$ we must necessarily have $|Z|=\mu.$

Now we argue that ${\rm cf}(X)=\mu.$ Letting $Y'$ be the range of a cofinal map $f:{\rm cf}(X)\to X$, the argument above with $Y'$ in place of $Y$ shows that we can assume that $f$ is is strictly increasing. The situation is now this: We have two maps $g:\mu\to X$ and $f:{\rm cf}(X)\to X,$ both strictly increasing and cofinal. Also, $\mu={\rm cf}(\mu),$ from item 2. By transfinite induction define subsequences of $f$ and $g$ as follows:  Given $(p_\alpha:\alpha<\beta),$ subsequence of $f,$ and $(q_\alpha:\alpha<\beta),$ subsequence of $g,$ if neither is cofinal, define $p_\beta$ to be the least element in the range of $f$ that is an upper bound for all the $p_\alpha$ and $q_\alpha$. Now define $q_\beta$ as the least member of the range of $g$ with $q_\beta\ge p_\beta.$ The construction stops when the sequences so built are cofinal in $X.$

If $X$ has a largest element, then $Z$ is a singleton, $\mu=1$, and also ${\rm cf}(X)=1$, so we may assume that this is not the case. It follows that the construction must stop at a limit ordinal $\beta$. But then $(p_\alpha:\alpha<\beta)$ witnesses that $\beta\ge{\rm cf}(X)$ (by definition of ${\rm cf}(X)$), so $\beta={\rm cf}(X).$ Similarly, $(q_\alpha:\alpha<\beta)$ is cofinal in $Z$ and $\beta\le\mu$ so $\beta=\mu$ by definition of $Z$ (or directly, from the sequence one easily sees that $\beta\le{\rm cf}(\mu)=\mu$ and there is a cofinal map $h:\beta\to\mu,$ so $\beta=\mu$).

We have shown that ${\rm cf}(X)=\beta=\mu.$ This completes the proof. ${\sf QED}$

Corollary 4. There is a strictly increasing cofinal map $f:{\rm cf}(\alpha)\to\alpha.$ $\Box$

Definition. An infinite cardinal $\kappa$ is regular iff ${\rm cf}(\kappa)=\kappa$. An infinite limit ordinal $\alpha$ is singular iff ${\rm cf}(\alpha)<\alpha.$

The following is immediate from Fact 1:

Corollary 5. ${\rm cf}(\kappa^+)=\kappa^+$ for all $\kappa.$

Proof. If $f:\alpha\to\kappa^+$ and $\alpha<\kappa^+,$ then $\alpha$ and all the $f(\beta)$ for $\beta\in\alpha$ have size at most $\kappa.$ But then $\sup({\rm ran}(f))=\bigcup_\beta f(\beta)\le\sum_\beta f(\beta),$ and $|\sum_\beta f(\beta)|\le\kappa$, by Fact 1. It follows that $f$ is bounded in $\kappa^+.$ ${\sf QED}$

Remark. That $\omega$ is regular does not require choice. Similarly, $\aleph_\omega$ has cofinality $\omega,$ and choice is not needed to establish this. On the other hand, Gitik showed that it is consistent with ${\sf ZF}$ that every uncountable initial ordinal has cofinality $\omega.$

If ${\rm cf}(\omega_1)=\omega$, then $\omega_1$ is the countable union of countable sets, since if $f:\omega\to\omega_1$ is cofinal, then $\omega_1=\bigcup_n f(n)$ and each $f(n)$ is a countable ordinal. On the other hand, if $X$ is a countable set of ordinals, then its order type is a countable ordinal, and it follows that any countable union of countable sets of ordinals has size at most $\omega_1.$ This is a curious situation, in that it is consistent that ${\rm cf}(\omega_1)={\rm cf}(\omega_2)=\dots=\omega$, and therefore there are countable unions of countable unions of countable sets that are not countable unions of countable sets, etc.

Homework problem 7. (${\sf ZF}$).

1. Assume that ${\mathbb R}$ is the countable union of countable sets. Show that ${\rm cf}(\omega_1)=\omega$
2. Assume that every countable union of countable sets is also a countable union of finite sets. Show that $\omega_1$ is regular.

Question. In ${\sf ZF}$, if every countable union of countable sets is also a countable union of finite sets, does it follow that every countable union of countable sets is countable?

Lemma 6. A cardinal $\kappa$ is regular if and only if it cannot be written as a union of fewer than $\kappa$ many sets, each of size less than $\kappa$.

Proof. If the right hand side holds, no map $f:\beta\to\kappa$ with $\beta<\kappa$ can be cofinal, so $\kappa$ is regular. Assume now that $\kappa=\bigcup_{i\in I}X_i$ where $|I|<\kappa$ and $|X_i|<\kappa$ for each $i\in I.$ Without loss, $I=|I|$ is an infinite cardinal, and it is least for which such a decomposition of $X$ is possible. Let $\kappa_i=|X_i|,$ and define a function $f: |I| \to \kappa$ by $f(\alpha)=\sum_{\beta<\alpha}\kappa_\beta,$ where sums denote ordinal addition. That the range of $f$ is indeed in $\kappa$ follows from our assumption on the minimality of $|I|.$ Notice that the ordinal $\sup({\rm ran}(f))=\sum_{\alpha<|I|}\kappa_\alpha$ has size $\kappa$, as the decomposition $X=\bigcup_{\alpha\in|I|}X_\alpha$ provides us with a straightforward bijection between $\kappa$ and $\sup({\rm ran}(f)).$ It follows that in fact $\sup({\rm ran})(f)=\kappa,$ i.e., $f$ is cofinal and therefore $\kappa$ is singular. ${\sf QED}$

We are ready to prove the first significant result.

Theorem 7. (König). Assume that $\kappa_i<\lambda_i$ for all $i\in I$. Then $\sum_i\kappa_i<\prod_i\lambda_i.$

Notice that $\sum_i\kappa_i=\sum_i\lambda_i$ is possible. For example, let $I=\omega$ and let each $\kappa_i$ be 1 and each $\lambda_i$ be 2. Then both sums equal $\omega.$

Proof. First we show that the inequality holds, and then we argue that it is strict. For the first part, we simply need to exhibit an injection from $\sqcup_{i\in I}\kappa_i=\bigcup_{i\in I}\kappa_i\times\{i\}$ into the collection of functions $\prod_{i\in I}\lambda_i.$ Simply assign to each $(\alpha,i)$ the function that is zero in each coordinate except the $i$-th one, where it takes the value $\alpha+1.$ Note that since $\alpha<\kappa_i<\lambda_i$, then indeed $\alpha+1<\lambda_i,$ and the function is well defined. Clearly, from the function we can recover $(\alpha,i),$ so the assignment is injective.

Now consider an arbitrary map $\varphi:\bigcup_i\kappa_i\times\{i\}\to\prod_i\lambda_i,$ and we need to exhibit a function not in its range. For this, notice that the assignment $\alpha\mapsto\varphi(\alpha,i)(i)$ maps $\kappa_i$ into $\lambda_i,$ and therefore there is some ordinal $g(i)\in\lambda_i$ not in its range. The function $g$ so defined is not in the range of $\varphi,$ since $g(i)\neq \varphi(\alpha,i)(i)$ for any $i\in I$ and any $\alpha\in\kappa_i.$ ${\sf QED}$

Note that at the heart of the proof just given there is again a diagonal argument.

### 6 Responses to 580 -II. Cardinal arithmetic

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