I've been asked the question:

For a function $ f $ from X $ \to $ Y, $ f^{-1}(f(A)) = A $ for every subset A of $ X $ if and only if: (a) $ f $ is injective (b) $ f $ is surjective (c) $ f $ is bijective.

A function is invertible only if it's bijective, and so I thought that the answer would be (c). But the correct answer, apparently, is (a).

Is there anything in the question that makes it clear that what's being asked is a left inverse, in which case it's sufficient for the function to be only injective? I'm guessing it has something to do with the statement "for every subset A of X" -- am I correct, or is there something else?

Also, if you were to specify "right inverse", how would you modify the question?

  • $\begingroup$ Why did you write "apparently the answer is (a)"? if $f(x)=y\in f(A)$ such that $x\in A$, shouldn't that be enough for $f$ being bijective? The only exception would be the restriction of 1-1: Exists an $y\in f(A)$ such that $f^{-1}(y) \ne x \in A$. $\endgroup$ – Lincon Ribeiro Feb 23 at 17:37
  • 1
    $\begingroup$ @LinconRibeiro See my answer - you cannot in fact conclude bijectivity from the OP's property. $\endgroup$ – Noah Schweber Feb 23 at 17:56

Remember that the notation "$f^{-1}(S)$" refers to the set $$\{x\in dom(f): f(x)\in S\}.$$ No assumptions on $f$ need to be made for this to make sense. In particular, using this notation does not presuppose that $f$ has an inverse.

Similarly, "$f(S)$" refers to the set $$\{x\in codom(f): \exists s\in S(f(s)=x)\}$$ (or, perhaps more simply, $\{f(s): s\in S\}$).

Unwinding definitions a bit, the problem here is just asking about the property

$(*)\quad$ For every set $A\subseteq dom(f)$ we have $$\{x\in dom(f): f(x)=f(a)\mbox{ for some $a\in A$}\}=A.$$

Now let's see why the answer to the question is (a) - that property $(*)$ holds iff $f$ is injective.

First, let's show that injectivity of $f$ implies $(*)$. It's helpful to note that we trivially have $A\subseteq \{x\in dom(f): f(x)=f(a)\mbox{ for some $a\in A$}\}$, so to show that property $(*)$ holds we only need to pay attention to the reverse inclusion. Suppose $f$ is injective and let $A\subseteq dom(f)$ be arbitrary. Let $x\in dom(f)$ such that $f(x)=f(a)$ for some $a\in A$. Since $f$ is injective, $x$ must actually be this $a$; that is, if $f(x)=f(a)$ for some $a\in A$ then $x\in A$ already. So the left hand side set is a subset of $A$, and per the comment above we're done.

Note that this is the surprising direction - that injectivity alone guarantees a property which at first glance may seem to be bijectivity. So before going on to the next point, make sure the argument above makes sense.

Now let's show that $(*)$ implies injectivity. Suppose for every $A\subseteq dom(f)$ we have $\{x\in dom(f): f(x)=f(a)\mbox{ for some $a\in A$}\}=A$. Fix $u,v\in dom(f)$ such that $f(u)=f(v)$; we want to show $u=v$. Let $A=\{u\}$. Then $\{x\in dom(f): f(x)=f(a)\mbox{ for some $a\in A$}\}$ contains $v$, so by our assumption $v\in A$; but $A=\{u\}$, so $v=u$.


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