Contents

foundations

# Contents

## Idea

The propositional axiom of choice is simply another name for the usual axiom of choice in dependent type theory, to distinguish with the untruncated version of the axiom of choice called the type theoretic axiom of choice.

It corresponds to the internal axiom of choice in categorical semantics and in set theory.

## Statement

The propositional axiom of choice is the statement that given types $A$ and $B$ and type family $x:A, y:B \vdash R(x, y)$, one can construct

$\mathrm{ac}_R:\left(\prod_{x:A} \exists y:B.R(x, y)\right) \to \left(\exists f:A \to B.\prod_{x:A} R(x, f(x))\right)$

The propositions as subsingletons interprets disjunction and existential quantification as the bracket type of the sum type and dependent sum type respectively, and leads to the usual statement of the axiom of choice translated to dependent type theory.

The equivalent form of the axiom of choice involving a unary type family $x:A \vdash P(x)$ instead of a binary type family $x:A, y:B \vdash R(x, y)$ states that one can construct

$\mathrm{ac}_R:\left(\prod_{x:A} [P(x)]\right) \to \left[\prod_{x:A} P(x)\right]$

This is the statement that any dependent product of any family of inhabited h-sets is inhabited.

## References

• Martin Hyland, p. 1 of: Variations on realizability: realizing the propositional axiom of choice, Mathematical Structures in Computer Science, Volume 12, Issue 3, June 2002, pp. 295 - 317 (doi:10.1017/S0960129502003651, pdf)

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