# The target of an epimorphism which is also a zero morphism is a zero object.

I'm stuck on the following problem.

In a category with a zero object; if an epimorphism is a zero morphism, then its target is a zero object.

## 1 Answer

Note that in a cat. with zero object, any initial object is the zero object (because all initial objects are isomorphic).

Thus, it is enough to show that the codomain $y$ of the said zero epimorphism $x \stackrel{0}\twoheadrightarrow y$ is initial. So given $z$, pick any two morphisms $\alpha, \beta: y \rightarrow z$ (also show that there is at least one such morphism). Now show that $\alpha$ is necessarily equal to $\beta$.

• Thank you for your answer. The part of that I am not able to prove is the uniqueness of the morphism from y to z. To prove this, I must use that $x \stackrel{0}\twoheadrightarrow y$ is an epimorphism. Yet, in order to do so, I must first show that for $\alpha \circ 0 = \beta \circ 0$. This is the part where I'm stuck. – Jules Pitcho Apr 12 '18 at 18:30
• @JulesPitcho Well, for any two pairs of objects $x', y'$, the zero map $x'\rightarrow y'$ is the unique map factorizing through the zero object. Do $\alpha \circ 0, \beta \circ 0$ factorize through the zero object? – Pavel Čoupek Apr 12 '18 at 18:42
• It's called a zero map because composing with it acts like multiplying by zero: absorbs it, trivializes it, etc.: making the composition a zero map. – Berci Apr 12 '18 at 23:43