Contents

category theory

# Contents

## Definition

###### Definition

In a category $C$ with zero object $0$ the zero morphism $0_{c,d} : c \to d$ between two objects $c, d \in C$ is the unique morphism that factors through $0$:

$0_{c,d} : c \to 0 \to d \,.$

More generally, in any category enriched over the closed monoidal category of pointed sets (with tensor product the smash product), the zero morphism $0_{c,d} : c \to d$ is the basepoint of the hom-object $[c,d]$.

###### Remark

In fact, an enrichment over pointed sets consists precisely of the choice of a ‘zero’ morphism $0_{c,d}:c\to d$ for each pair of objects, with the property that $0_{c,d} \circ f = 0_{b,d}$ and $f\circ 0_{a,b} = 0_{a,c}$ for any morphism $f:b\to c$. Such an enrichment is unique if it exists, for if we are given a different collection of zero morphisms $0'_{c,d}$, we must have

$0'_{c,d} = 0'_{c,d} \circ 0_{c,c} = 0_{c,d}$

for any $c,d$. Thus, the existence of zero morphisms can be regarded as a property of a category, rather than structure on it. (To be more precise, it is an instance of property-like structure, since not every functor between categories with zero morphisms will necessarily preserve the zero morphisms, although an equivalence of categories will.)

## Examples

See at zero object for examples.

Last revised on April 19, 2018 at 02:54:44. See the history of this page for a list of all contributions to it.