# Left inverse in $F_{A}$ iff injective proof.

Let A be a non-empty set and $f : A → A$ be a function.

Prove that f has a left inverse in $F_{A}$ if and only if f is injective (one-to-one).

$\leftarrow$ assume f is injective then $\forall x\in A \space \space \space \space \space \space \space \space \space f(x) \in A$ such that if $f(x)=f(y)$ then $x=y$

something something $g(f(x)) = x \space \space \space \space \forall x\in A$

$\rightarrow$ assume f has a left inverse in $F_{A}$ then $\forall x\in A$

$g(f(x)) = x$ something says that x must be one to one?

Im really confused by this question First of all f must be a bijection if it is one to one from $A \to A$ is it not?

Can someone help me out with this proof?

• Are any functions $f : \{0\} \to \{1, 2\}$ one-to-one? Are they bijections too? Could you write down any left inverses for any of these one-to-one functions? – pjs36 Jan 16 '17 at 4:33
• What is $F_A{}$? – Eric Wofsey Jan 16 '17 at 4:34
• it has a left inverse not a bijection not one to one. – Faust Jan 16 '17 at 4:34
• $F_{A}$ is the set of functions that map A to A – Faust Jan 16 '17 at 4:35
• Possible duplicate of Left inverse iff injective; right inverse iff surjective – Javier Apr 8 at 13:57

Suppose $f$ has a left inverse $g$.
Suppose $f(x)=f(y)$ for some $x,y\in A$.
Then $gf(x)=gf(y)$.
Since $gf=i_A$, we have $x=y$ and hence $f$ is injective.

Suppose $f$ is injective. Let $a_0$ be a fixed element in $A$.
Define $g:A\rightarrow A$ by the rules:
For $x\in A$
(i) If there is an element $y\in A$ such that $f(y)=x$, then $g(x)=y$.
(ii) If no such element $y$ exists in $A$, then $g(x)=a_0$.

For case (i), the element $y$ is unique as if $g(x)=y_1$ and $g(x)=y_2$, then $f(y_1)=x=f(y_2)$ which implies $y_1=y_2$.
For case (ii), $g(x)=a_0$ is also unique by the way we define it.
So $g$ is a function from $A$ to $A$.

For $a\in A$, $gf(a)=a$ as $f(a)=x\in A$ implies $g(x)=a$ by (i).
Hence $g$ is a left inverse of $f$.

• ii) was the hard side. – Faust Jan 16 '17 at 4:45
• $\forall y \in A g(f(y))=x$ so g compose f is the same as $I (y) \forall y \in A$ – Faust Jan 16 '17 at 4:47
• @Faust7 for all $y\in A$, $gf(y)=y$, so $gf=i_A$, the identity function from $A$ to $A$. – Alan Wang Jan 16 '17 at 4:51
• Yes that was what i was trying to say but couldn't :( Thank you very much your explanation was flawless. – Faust Jan 16 '17 at 4:51

First of all, a one-to-one set function $f: A \to A$ certainly need not be a bijection; consider the multiplication by two map $\times 2: \mathbb{Z} \to \mathbb{Z}$. This is injective, but no odd numbers are in the image.

Now for your main question, assume $f$ is injective. Then define a left inverse $g$ by setting $g(x) = f^{-1}(x)$ wherever $f^{-1}(x)$ is defined. Because $f$ is injective, there is only one possible value for $f^{-1}(x)$, whenever it exists. Thus, $g$ is a left inverse of $f$ (because $g(f(x)) = x$ by definition). In the other direction, suppose that $g$ is a left inverse to $f$, i.e. for all $x \in A$, $g(f(x)) = x$. Now suppose that $f(x) = f(y)$. Applying $g$ to both sides, we see that $g(f(x)) = g(f(y))$, and since $g$ is a left inverse, this gives $x = y$. Thus $f$ is injective.

Each of the two answers in this article have flaws in their proofs that every injective function has a left inverse, so here we set the record straight.

The accepted answer doesn't account for the fact that $$A$$ might be empty (this is really hair splitting). The answer by Reuben Stern only constructs a left inverse when the function $$f$$ is a bijection.

If $$f: A \to A$$ is injective, the converse relation $$f^{-1}$$ will define a bijective function

$$f^{-1}: f(A) \to A$$

satisfying $$f^{-1} \circ f = id_A$$.

If $$f(A) = A$$ then $$f^{-1} \in F_A$$ and we are done.

If $$f$$ is not surjective let $$a_0 \in A \setminus f (A)$$ and define the function $$g: A \to A$$ by

$$g(a) = \left\{\begin{array}{lr} f^{-1}(a)\, \;\;\;\text{ |} & \text{for } a \in f(A)\\ a_0 \quad \quad \,\,\;\; \text{ |} & \text{for } a \in A \setminus f (A) \end{array}\right\}$$

It is easily checked that $$g \circ f = id_A$$.