arithmetic with cardinals. a kind of transfinite arithmetic
For $S$ a set, write ${|S|}$ for its cardinality. Then the standard operations in the category Set induce arithmetic operations on cardinal numbers:
For $S_1$ and $S_2$ two sets, the sum of their cardinalities is the cardinality of their disjoint union, the coproduct in $Set$:
More generally, given any family $(S_i)_{i: I}$ of sets indexed by a set $I$, the sum of their cardinalities is the cardinality of their disjoint union:
Likewise, the product of their cardinalities is the cardinality of their cartesian product, the product in $Set$:
More generally again, given any family $(S_i)_{i: I}$ of sets indexed by a set $I$, the product of their cardinalities is the cardinality of their cartesian product:
Also, the exponential of one cardinality raised to the power of the other is the cardinality of their function set, the exponential object in $Set$:
In particular, we have $2^{|S|}$, which (assuming the law of excluded middle) is the cardinality of the power set $P(S)$. In constructive (but not predicative) mathematics, the cardinality of the power set is $\Omega^{|S|}$, where $\Omega$ is the cardinality of the set of truth values.
The usual way to define an ordering on cardinal numbers is that ${|S_1|} \leq {|S_2|}$ if there exists an injection from $S_1$ to $S_2$:
Classically, this is almost equivalent to the existence of a surjection $S_2 \to S_1$, except when $S_1$ is empty. Even restricting to inhabited sets, these are not equivalent conditions in constructive mathematics, so one may instead define that ${|S_1|} \leq {|S_2|}$ if there exists a subset $X$ of $S_2$ and a surjection $X \to S_1$. Another alternative is to require that $S_1$ (or $X$) be a decidable subset of $S_2$. All of these definitions are equivalent using excluded middle.
This order relation is antisymmetric (and therefore a partial order) by the Cantor–Schroeder–Bernstein theorem (proved by Cantor using the well-ordering theorem, then proved by Schroeder and Bernstein without it). That is, if $S_1 \hookrightarrow S_2$ and $S_2 \hookrightarrow S_1$ exist, then a bijection $S_1 \cong S_2$ exists. This theorem is not constructively valid, however.
The well-ordered cardinals are well-ordered by the ordering $\lt$ on ordinal numbers. Assuming the axiom of choice, this agrees with the previous order in the sense that $\kappa \leq \lambda$ iff $\kappa \lt \lambda$ or $\kappa = \lambda$. Another definition is to define that $\kappa \lt \lambda$ if $\kappa^+ \leq \lambda$, using the successor operation below.
The successor of a well-ordered cardinal $\kappa$ is the smallest well-ordered cardinal larger than $\kappa$. Note that (except for finite cardinals), this is different from $\kappa$'s successor as an ordinal number. We can also take successors of arbitrary cardinals using the operation of Hartog's number?, although this won't quite have the properties that we want of a successor without the axiom of choice.
It is traditional to write ℵ${}_0$ for the first infinite cardinal? (the cardinality of the natural numbers), $\aleph_1$ for the next (the first uncountable cardinality), and so on. In this way every cardinal (assuming choice) is labeled $\aleph_\mu$ for a unique ordinal number $\mu$, with $(\aleph_\mu))^+ = \aleph_{\mu^+}$.
For every cardinal $\pi$, we have $2^\pi \gt \pi$ (this is sometimes called Cantor's theorem). The question of whether $2^{\aleph_0} = \aleph_{1}$ (or more generally whether $2^{\aleph_\mu} = \aleph_{\mu^+}$) is called Cantor’s continuum problem; the assertion that this is the case is called the (generalized) continuum hypothesis. It is known that the continuum hypothesis is undecidable in ZFC.
For every transfinite cardinal $\pi$ we have (using the axiom of choice) $\pi + \pi = \pi$ and $\pi \cdot \pi = \pi$, so addition and multiplication are idempotent.
Lecture notes include