nLab product extensionality

Idea

The counterpart to function extensionality but for products/pairs of elements $(a, b)$. Product extensionality characterizes the equality of elements in cartesian products in set theory and product types in type theory.

Definition

In set theory

Product extensionality states that for all elements $a \in A$, $a' \in A$, $b \in B$, and $b' \in B$, $a = a'$ and $b = b'$ if and only if $(a, b) = (a', b')$. Product extensionality is sometimes assumed as an axiom in material set theory to make the pairing structure into an ordered pairing structure $(a, b)$.

In type theory

In intensional type theory, equality is represented by the identity type, and furthermore, there might be more than one element of the identity type in intensional type theory, so a naive translation of product extensionality into intensional type theory doesn’t result in the right statement.

Instead, product extensionality is the statement that given types $A$ and $B$, the binary function application to identities $\mathrm{apbinary}_{(-,-)}(a, a', b, b')$ of the function $a:A, b:B \vdash (a, b):A \times B$ defined in the introduction rule for the product type is an equivalence of types for all elements $a:A$, $a':A$ and $b:A$, $b':A$:

$$\mathrm{prodext}(a, a', b, b'):\mathrm{isEquiv}(\mathrm{apbinary}_{(-,-)}(a, a', b, b'))$$

Judgmental product extensionality

One could replace the equivalence of types above with a judgmental equality of types, resulting in judgmental product extensionality, that for all types $A$ and $B$ and elements $a:A$, $a':A$, $b:B$, and $b':B$,

$$(a, b) =_{A \times B} (a', b') \equiv (a =_A a') \times (b =_B b')$$

Judgmental product extensionality holds in cubical type theory and higher observational type theory.

Relation to dependent product extensionality

Product extensionality is equivalent to dependent product extensionality for dependent products indexed by the two-valued type $\mathbb{2}$. Indeed, given types $A$ and $B$, we can define a type family $C$ indexed by $\mathbb{2}$ as $C(0) \coloneqq A$ and $C(1) \coloneqq B$. Then $A \times B$ is the same as $\prod_{x:\mathbb{2}} C(x)$.

We define $c:\prod_{x:\mathbb{2}} C(x)$ and $c':\prod_{x:\mathbb{2}} C(x)$ by $c(0) \coloneqq a$, $c(1) \coloneqq b$, $c'(0) \coloneqq a'$, $c'(1) \coloneqq b'$.

There is a canonical function

$$\mathrm{idstodepprodid}(c, c'):(c(0) =_{C(0)} c'(0)) \times (c(1) =_{C(1)} c'(1)) \to (c =_{\prod_{x:\mathbb{2}} C(x)} c')$$

inductively defined by

$$\mathrm{idstodepprodid}(c, c)(\mathrm{refl}_{C(0)}(c(0)), \mathrm{refl}_{C(1)}(c(1))) \equiv \mathrm{refl}_{\prod_{x:\mathbb{2}} C(x)}(c):\Omega(\prod_{x:\mathbb{2}} C(x), c)$$

where $\Omega(A, a)$ is the loop space type $a =_A a$ of $A$ at $a:A$.

We could also make $(c(0) =_{C(0)} c'(0)) \times (c(1) =_{C(1)} c'(1))$ into a dependent product as well, as $\left(\prod_{x:\mathbb{2}} (c(x) =_{C(x)} c'(x))\right)$, making the above function

$$\mathrm{idstodepprodid}(c, c'):\left(\prod_{x:\mathbb{2}} (c(x) =_{C(x)} c'(x))\right) \to (c =_{\prod_{x:\mathbb{2}} C(x)} c')$$

Product extensionality is the statement that the function $\mathrm{idstodepprodid}(c, c')$ is an equivalence of types for all elements $c:\prod_{x:\mathbb{2}} C(x)$ and $c':\prod_{x:\mathbb{2}} C(x)$:

$$\mathrm{prodext}(c, c'):\mathrm{isEquiv}(\mathrm{idstodepprodid}(c, c'))$$

See also

• extensionality
• tuple extensionality

References

For judgmental product extensionality in higher observational type theory, see

• Mike Shulman, Towards a Third-Generation HOTT Part 1 (slides, video)

For a proof that the given function above between identity types is a quasi-inverse function in intensional type theory, see section 2.6:

• Homotopy Type Theory: Univalent Foundations of Mathematics, The Univalent Foundations Project, Institute for Advanced Study, 2013. (web, pdf)