Contents

foundations

# Contents

## Idea

In the foundations of mathematics, the axiom of infinity asserts that infinite sets exist. Infinite sets cannot be constructed from finite sets, so their existence must be posited as an extra axiom. Further axioms in this vein which assert the existence of even larger sets that cannot be constructed from smaller ones are called large cardinal axioms.

## Statements

One common form of the axiom of infinity says that the particular set $N$ of natural numbers exists. In material set theory this often takes the form of asserting that the von Neumann ordinal number $\omega$ exists, where $\omega$ is characterized as the smallest set such that $\emptyset\in\omega$ and whenever $a\in \omega$ then $a\cup \{a\}\in \omega$. In structural set theory the usual form of the axiom of infinity is the existence of a natural numbers object.

In dependent type theory, the axiom of infinity for a Tarski universe is given by the element

$\mathrm{axinf}_U:\sum_{\mathbb{N}:U} \sum_{0:T(\mathbb{N})} \sum_{s:T(\mathbb{N}) \to T(\mathbb{N})} \prod_{C:\mathbb{N} \to U} \prod_{c_0:T(C(0))} \prod_{c_s:\prod_{x:\mathbb{N}} T(C(x)) \to T(C(s(x)))} \exists!c:\prod_{x:\mathbb{N}} T(C(x)).(c(0) =_{T(C(0))} c_0) \times \prod_{x:\mathbb{N}} (c(s(x)) =_{T(C(s(x)))} c_s(c(x)))$

which states that there is a natural numbers type in the universe.

There is an alternate way to express the axiom of infinity in a Tarski universe, as the axiom of resizing the set truncation of the type of finite types in $U$, since $\mathrm{isFinite}$ and set truncations are definable from the type of propositions in $U$, $\sum_{A:U} \mathrm{isProp}(A)$, but they are all usually large, and so have to be resized to be small:

$\mathrm{axinf}_U:\sum_{\mathbb{N}:U} T(\mathbb{N}) \simeq \left[\sum_{A:U} \mathrm{isFinite}(T(A))\right]_0$

In the form of an NNO, the axiom of infinity generalises to the existence of inductive types or W-types. These can be constructed from a NNO if power sets exist, but in predicative theories they can be added as additional axioms.

One could also posit the existence of the set of extended natural numbers instead of the set of natural numbers, as the set of extended natural numbers have countably infinite cardinality and is the categorical dual of the natural numbers in Set, a terminal coalgebra for the endofunctor $F(X) = 1 + X$ in Set. This generalises to the existence of coinductive types or M-types, which can be added as additional axioms.

One could also posit the existence of a universe $\mathcal{U}$ that satisfies the axiom of finiteness (see below), or a type of finite types.

## Alternatives

Broadly speaking, finite mathematics is mathematics that does not use or need the axiom of infinity; a finitist is an extreme breed of constructivist that believes that mathematics is better without the axiom of infinity, or even that this axiom is false.

A more extreme case is to deny the axiom of infinity with an axiom of finiteness: every set is finite. There is one of these for every definition of ‘finite’ given on that page; here is the strongest stated directly in terms of set theory as an axiom of induction:

• Any property of sets that is invariant under isomorphism and holds for the empty set must hold for all sets if, whenever it holds for a set $X$, it holds for the disjoint union $X \uplus \{*\}$.

In material set theory, this is equivalent given the axiom of foundation (which guarantees that $X$ and $\{X\}$ are disjoint):

• Any property of sets that holds for the empty set must hold for all sets if, whenever it holds for a set $X$, it holds for the union $X \cup \{X\}$.

In higher categorical terms, the above axiom of finiteness could be stated as follows: Set is an initial algebra of the 2-endofunctor $F(X) \cong X \coprod 1$ in the (2,1)-category Grpd.

In dependent type theory, given a Tarski universe $(U, T)$ that is closed under the empty type, the unit type, and sum types, the axiom of finiteness for the universe states that

• For all type families $A:U \vdash C(A)$ such that $T(A) \simeq T(B)$ implies that $C(A) \simeq C(B)$, elements $c_0:C(\mathbb{0})$ and dependent functions $c_s:\prod_{A:U} C(A) \to C(A + \mathbb{1})$, there exists a unique dependent function $c:\prod_{A:U} C(A)$ such that $c(\mathbb{0}) =_{C(\mathbb{0})} c_0$ and for all $A:U$, $c(A + 1) =_{C(A + 1)} c_s(c(A))$.

In dependent type theory with dependent product types, dependent sum types, identity types, function extensionality, and a type of all propositions, the axiom of finiteness for the entire type theory is an axiom schema which states that given a type $A$, one could derive a witness that the type is a finite type:

$\frac{\Gamma \vdash A \; \mathrm{type}}{\Gamma \vdash \mathrm{finWitn}_A:\mathrm{isFinite}(A)}$

where

$\mathrm{isFinite}(A) \equiv \begin{array}{c} \prod_{S:(A \to \mathrm{Prop}) \to \mathrm{Prop}} (((\lambda x:A.\bot) \in S) \times \prod_{P:A \to \mathrm{Prop}} \prod_{Q:A \to \mathrm{Prop}} (P \in S) \\ \times (\exists!x:A.x \in Q) \times (P \cap Q =_{A \to \mathrm{Prop}} \lambda x:A.\bot) \to (P \cup Q \in S)) \to ((\lambda x:A.\top) \in S) \end{array}$

The membership relation and the subtype operations used above are defined in the nLab article on subtypes.

In particular, the axiom of finiteness for the entire type theory implies the principle of excluded middle for the type of all propositions, since the only finite propositions are the decidable propositions. Furthermore, the axiom of finiteness implies that the type theory is a set-level type theory because every finite type is an h-set.

In relation to classifying toposes: