Contents

Idea

In dependent type theory, there are two notions of equality,

1. strict or judgmental equality,

2. weak or typal equality.

Accordingly there are two corresponding notions of the inverse function of a function $f:A \to B$:

1. There are the various notions of weak inverse functions, such as quasi-inverse functions and coherent inverse functions, which is a function $f^{-1}:B \to A$ with homotopies $h:\prod_{x:A} f^{-1}(f(x)) =_A x$ and $k:\prod_{y:B} f(f^{-1}(y)) =_B y$ and possibly other conditions and structure.

2. There is the notion of strict inverse function, which is a function $f^{-1}:B \to A$ with judgmental equalities $x:A \vdash f^{-1}(f(x)) \equiv x:A$ and $y:B \vdash f(f^{-1}(y)) \equiv y:B$. The other structures are unimportant since non-trivial paths for weak inverse functions reduce to reflexivity for $x:A$ or $y:B$ for strict inverse functions.

 Examples

• If copy types are defined using judgmental computation rules and uniqueness rules, then the constructor and eliminator functions are strict inverse functions of each other.

• In Mike Shulman‘s higher observational type theory the $\mathrm{idtoequiv}$ function for the univalent universes has a strict inverse function.

See also

• judgmental equality

• inverse function