Contents

category theory

# Contents

## Definition

A De Morgan Heyting category is a Heyting category $C$ (such as a topos or pretopos) in which for every object $X \in \mathrm{Ob}(C)$ the pseudocomplement of the intersection of two subobjects $A \in \mathrm{Sub}(X)$ and $B \in \mathrm{Sub}(X)$ is the union of the pseudocomplements of $A$ and $B$, $((A \cap B) \implies \emptyset) = (A \implies \emptyset) \cup (B \implies \emptyset)$. Equivalently, it is a Heyting category $C$ in which for every object $X \in \mathrm{Ob}(C)$ the union of the pseudocomplement of every subobject $A \in \mathrm{Sub}(X)$ and the double pseudocomplement of $A$ is an improper subobject, $A \in \mathrm{Sub}(X)$, $(A \implies \emptyset) \cup ((A \implies \emptyset) \implies \emptyset) = \Im(X)$.

Therefore, the subobject poset $Sub(X)$ of any object $X \in \mathrm{Ob}(C)$ is a De Morgan Heyting algebra. De Morgan's law and weak excluded middle hold in the internal logic of $C$.

In addition, every De Morgan Heyting category $C$ is a first order De Morgan Heyting hyperdoctrine given by the subobject poset functor $\mathrm{Sub}:\mathcal{C}^{op} \to DeMorgHeytAlg$.