nLab isomorphism in a Segal type

Definition

In simplicial type theory, given a Segal type $A$ and terms $x \colon A$ and $y \colon A$, a “morphism” (term of hom type) $f \colon \mathrm{hom}_A(x, y)$ is an isomorphism if there exist morphisms $g:\mathrm{hom}_A(y, x)$ and $h:\mathrm{hom}_A(y, x)$ such that $g \circ f = \mathrm{id}_x$ and $f \circ h = \mathrm{id}_y$

$$\mathrm{isIso}(f) \coloneqq \left(\sum_{g:\mathrm{hom}_A(y, x)} g \circ f = \mathrm{id}_x\right) \times \left(\sum_{h:\mathrm{hom}_A(y, x)} f \circ h = \mathrm{id}_y\right)$$

The type of all isomorphisms between $x$ and $y$ in $A$ is defined as

$$\big(x \cong_A y\big) \;\;\coloneqq\;\; \sum_{ \mathclap{ f \colon \mathrm{hom}_A(x, y) } } \mathrm{isIso}(f)$$

Related concepts

• hom type

• Segal type

• Rezk type

• isomorphism

• equivalence in an (∞,1)-category

References

• Emily Riehl, Michael Shulman, A type theory for synthetic $\infty$-categories $[$arXiv:1705.07442$]$