[Math] Axiom of Choice and Right Inverse

alternative-proofaxiom-of-choicefunctions

I read an Theorem that states:

Let $A$ and $B$ be non-empty sets, and let $f:A \to B$ be a function,
then the function $f$ has a right inverse if and only if $f$ is
surjective.

The Theorem proof uses Axiom of Choice. My question is, if Axiom of Choice is not true, then is this Theorem false? My question is because I tried to find a proof that not uses the axiom but I did not found it and this result is very important because it implies, with another statement, that a function $f$ has an inverse if and only if $f$ is bijective.

Best Answer

This is a very delicate point about the context of domain and codomain, which in set theory exist as an external properties we give functions, rather than internal properties of them (as in category theory).

This means that we first fix some domain and codomain and we can talk about functions from $A$ to $B$. Now we can say that $f$ is surjective if its range is all $B$. Note that without this context there is no codomain, and every function is onto its range, therefore the term "surjective" is meaningless.

Now when we say that a function $f$ has a right inverse we mean that there is $g\colon B\to A$ such that $f\circ g=\text{id}_B$. When we say that $f$ has a left inverse we mean there is such $g\colon B\to A$ for which $g\circ f=\text{id}_A$. Without the axiom of choice we can prove that if a function has a right inverse then it is surjective, and that $f$ is injective if and only if it has a left inverse.

The axiom of choice is equivalent to saying that whenever $A$ and $B$ are two sets, the $f$ is a surjective function if and only if it has a right inverse. But we had set the context first. Of course we can omit it, because we are talking about any set, so we may as well replace $B$ by the range of $f$. But some context must be set for otherwise the term "surjective" is meaningless.

On the other hand, the very basic theorem that $f\colon A\to B$ has an inverse if and only if it is a bijection has nothing to do with the axiom of choice. The statement is really saying that $f$ is a bijection if and only if there is some $g\colon B\to A$ which is both right and left inverse of $f$. From the above remarks you can see that without the axiom of choice if there is an inverse (both left and right, that is) then the function is a bijection; but if it is a bijection then we can construct its left inverse and show that it is also a right inverse.