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I need help on the following problems:

QN1 Consider the map $f:(\Bbb R\times\Bbb R,\tau)\to(\Bbb R,\gamma)$ given by $f(x\times y)=|x-y|$,where $\tau$ is the product topology and $\gamma$ is the usual topology. Find $f(1),f(0),f^{-1}(0)$ and $f^{-1}[0,1)$.

for this problem ,I don’t know how to use the given definition to find say $f(1)$, also I get difficult how to come with $f^{-1}$.

Another one is someone to help me, I read somewhere that the set of rational number $\Bbb Q$ as a subspace of $\Bbb R$ (with usual topology) does not have discrete topology since the point $0$ is not an open set in $\Bbb Q$. Why this argument true?, For me I know that $0$ is rational. Thanks

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f(1) doesn't seem to make sense to me. (Is R the set of real numbers?) The topology on Q is given by 'open balls' (= open intervals) on Q. Can you find a radius r and a midpoint q such that the open ball of radius r about the point q (i.e. the interval (q-r, q+r)) contains 0 and no other points? –  Billy May 11 '13 at 20:05
@Billy:yes R is the set of real number,Thanks –  user77362 May 11 '13 at 20:19

1 Answer 1

The number $1$ is not (at least not without further conventions) an element of $\mathbb R\times \mathbb R$, hence I understand why you have problems finding out what $f(1)$ should be.

For $f^{-1}$, you need to find all $(x,y)$ such that $|x-y|$ ...

For the totally unrelated about $\mathbb Q$, note that $\frac1n\to 0$.

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:Yes It confuse me how to do,I need help how to work out that? Is there a way to write 1 in the form XxY?.Thanks –  user77362 May 11 '13 at 20:42
No - that's Hagen's point (and mine). The question, as it stands, doesn't make sense. If there's extra information you're not telling us, please do; otherwise, the question is unclear. –  Billy May 11 '13 at 20:54
Owkay,Let me find out.Thanks –  user77362 May 11 '13 at 21:04

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