Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

I'm trying to practice my proof-writing skills. At the moment I trying to prove the following proposition:

For every one-to-one function $\: f:X \to Y \:$, and sequence of sets $\: B_n \subseteq X \:$,

$$\: f \left[\bigcap_{n=0}^\infty B_n \right] = \bigcap_{n=0}^\infty f \left[B_n\right].$$

I was hoping to get some feedback on the proof. I suppose both in terms of content and form. Thanks!


First, for some arbitrary $y \in Y$, suppose that $y \in f\left[ \bigcap_{n=0}^\infty B_n\right]$.

Then, by definition, $\exists x \in \bigcap_{n=0}^\infty B_n$ such that $f(x) = y$.

And, because we have an intersection, it follows that $\exists x : \forall n\, (x \in B_n)$.

In that case, $y \in f\left[B_n\right]$ for each $n$.

And therefore, $y \in \bigcap_{n=0}^\infty f\left[B_n\right]$.

So we've shown that $y \in f\left[ \bigcap_{n=0}^\infty B_n\right] \implies y \in \bigcap_{n=0}^\infty f\left[B_n\right]$.

Now, assuming that $f$ is one-to-one, suppose that we have $y \in f\left[B_n\right]$ for each $n$.

So for all $n$, there is an $x \in X$ such that $f(x) = y$.

But if $f$ is an injection then, by definition, there is only one such $x$.

It follows that for each $n$, this unique $x$ is in $B_n$, that is $x \in \bigcap_{n=0}^\infty B_n$.

And $x \in \bigcap_{n=0}^\infty B_n \implies y \in f\left[ \bigcap_{n=0}^\infty B_n\right] $.

So we've shown that $y \in \bigcap_{n=0}^\infty f\left[B_n\right] \implies y \in f\left[ \bigcap_{n=0}^\infty B_n\right] $.

So the implication goes in both directions, which is what we needed to show.

End proof.

Thanks in advance for your comments. I really appreciate it.

share|improve this question
The writing could be tightened up just a bit, but it’s correct and reasonably clear. –  Brian M. Scott Feb 2 '13 at 23:19
@WilloW In the first inclusion you took $y\in Y$ first and you supposed it was in LHS set of the equality. Despite not being incorrect, there's no need for that. You can simply take $y$ in the LHS set from the get go. –  Git Gud Feb 2 '13 at 23:24
Thanks for the comments! Really appreciate it. –  WilloW Feb 3 '13 at 17:19

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.