# Contents

## Definition

### In first order logic with equality

#### Uniqueness up to equality

In first order logic with equality, given a predicate $P$ on a type $T$ with equality, the uniqueness quantifier of $P$, denoted $\exists!\, x\colon T, P(x)$, is defined in terms of the universal and existential quantifiers as

$\exists!\, x\colon T, P(x) \;\equiv\; \exists\, x\colon T, P(x) \wedge \forall\, y\colon T, P(y) \Rightarrow (x = y) .$

The intended interpretation is that $\exists!\, x\colon T, P(x)$ is true iff $P(a)$ is true for exactly one element $a$ of $T$.

#### Uniqueness up to isomorphism

Sometimes, we want to use a weaker notion of equivalence than strict equality, such as isomorphism $x \cong y$. The uniqueness up to isomorphism quantifier of $P$, denoted $\exists!_\cong\, x\colon T, P(x)$, is defined in terms of the universal and existential quantifiers as

$\exists!_\cong\, x\colon T, P(x) \;\equiv\; \exists\, x\colon T, P(x) \wedge \forall\, y\colon T, P(y) \Rightarrow (x \cong y).$

The intended interpretation is that $\exists!_\cong\, x\colon T, P(x)$ is true iff $P(a)$ is true for exactly one element $a$ of $T$ up to isomorphism.

Uniqueness up to isomorphism quantifiers are important in category theory, where the relevant notion of sameness is isomorphism rather than strict equality. It is also important in foundational set theories where the type of sets does not have equality, such as some presentations of SEAR and ETCS.

### In dependent type theory

In dependent type theory, given a type $T$ and a type family $x:T \vdash P(x)$, the uniqueness quantifier is a type defined as

$\exists!\, x\colon T. P(x) \coloneqq \mathrm{isContr}\left(\sum_{x:T} P(x)\right)$

which indicates that the dependent sum type $\sum_{x:T} P(x)$ is a contractible type, which is only the case for a family of type if every dependent type is a mere proposition and, for exactly one element $x:T$ up to identity, the type $P(x)$ is inhabited.

## Usages

### Defining exclusive disjunction

In dependent type theory, given two mere propositions $P$ and $Q$, by descent or large elimination? of the boolean domain, one can construct a boolean-indexed family of propositions

$x:\mathrm{bool} \vdash \mathrm{rec}_\mathrm{bool}^{P, Q}(x)$
$\beta_\mathrm{bool}^{P, Q}(0):\mathrm{rec}_\mathrm{bool}^{P, Q}(0) \simeq P \quad \beta_\mathrm{bool}^{P, Q}(1):\mathrm{rec}_\mathrm{bool}^{P, Q}(1) \simeq Q$

in the case for descent for booleans, or with judgmental equality of types

$\mathrm{rec}_\mathrm{bool}^{P, Q}(0) \equiv P \quad \mathrm{rec}_\mathrm{bool}^{P, Q}(1) \equiv P$

in the case for large elimination for booleans.

The uniqueness quantifier of the above family of propositions is the exclusive disjunction of $P$ and $Q$:

$P \underline{\vee} Q \coloneqq \exists!x:\mathbb{2}.\mathrm{rec}_\mathrm{bool}^{P, Q}(x)$

### Bijections and equivalences

The uniqueness quantifier is used in the definition of a bijection in set theory and an equivalence in type theory, where one defines a function $f:A \to B$ to be a bijection or equivalence if for all $y:B$ the there is a unique $x:A$ such that $f(x) =_B y$

$\mathrm{isEquiv}(f) \coloneqq \forall y:B.\exists! x:A.f(x) =_B y$

In dependent type theory, this is the same as defining a family of elements $x:A \vdash f(x):B$ to be an equivalence if it comes with a family of elements

$y:B \vdash \epsilon(f)(y):\exists! x:A.f(x) =_B y$

The inverse of an equivalence is given by the family of elements $y:B \vdash \epsilon_A(\epsilon(f)(y)):A$, where $\epsilon_A$ is defined in the elimination rules for uniqueness quantifiers in dependent type theory.

### Anafunctions

Similarly, uniqueness quantifications is used in the definition of an anafunction, where one defines a relation or correspondence $x:A, y:B \vdash R(x, y)$ to be an anafunction if for all $x:A$ there is a unique $y:B$ such that $R(x, y)$

$\mathrm{isAnafunc}(R) \coloneqq \forall x:A.\exists! y:B.R(x, y)$

### Univalent universes

Uniqueness quantifiers are also used to define univalent universes. A Russell universe $U$ is a univalent universe if for all elements $A:U$ there is a unique $B:U$ such that $A \simeq B$:

$\mathrm{isUnivalent}(U) \coloneqq \forall A:U.\exists! B:U.A \simeq B$

Similarly, a Tarski universe $(U, T)$ is a univalent universe if for all elements $A:U$ there is a unique $B:U$ such that $T(A) \simeq T(B)$:

$\mathrm{isUnivalent}(U, T) \coloneqq \forall A:U.\exists! B:U.T(A) \simeq T(B)$