nLab
Heyting category

Definition

A Heyting category (called a logos in Freyd-Scedrov) is a coherent category in which each base change functor $f^*:Sub(Y)\to Sub(X)$ has a right adjoint $\forall_f$ (the universal quantifier, in addition to the left adjoint existential quantifier that exists in any regular category).

It follows that each poset of subobjects $Sub(X)$ is a Heyting algebra and the base-change functors $f^*$ are Heyting algebra homomorphisms. The Heyting implication can be defined as follows: if $A$ and $B$ are subobjects of $X$, then we can define $(A\Rightarrow B) \in Sub(X)$ as $\forall_i(A\cap B) \hookrightarrow X$, where $i\colon A\hookrightarrow X$ is the subobject inclusion and we take $A \cap B\in Sub(A)$. Any Heyting category has an internal logic which is full (typed) first-order intuitionistic logic. The extra right adjoint $\forall_f$ provided by the above axiom gives the universal quantifier in this logic.

A Heyting functor is a coherent functor between Heyting categories which additionally preserves the right adjoints $\forall_f$. Such functors also preserve the internal interpretation of first-order logic.

Examples

Any topos, and in fact any well-powered infinitary coherent category, is a Heyting category. Any Boolean category is also a Heyting category.

References

• Peter Freyd, Andre Scedrov, Categories, Allegories