# f: [0,1) -> R

• Oct 24th 2006, 06:59 AM
srt12
f: [0,1) -> R
Hi there,

I wonder if you can think of any bijective function that maps [0,1) interval into R?

In case of (0,1) I've done the following: y=tan(pi*x - pi/2), but I have no idea how to deal with [0,1).

Thanks in advance for any help,
Lucas
• Oct 24th 2006, 07:33 AM
earboth
Quote:

Originally Posted by srt12
Hi there,

I wonder if you can think of any bijective function that maps [0,1) interval into R?...

Hi,

try this one: $\displaystyle f(x)=\frac{\sqrt{x-x^2}}{x-\frac{1}{2}}$

Unfortunately this function maps $\displaystyle [0,1]\mapsto \mathbb{R}$

But may be you find a restriction so you can use your interval.

EB
• Oct 24th 2006, 07:39 AM
TD!
Or: try mapping [0,1) to (0,1), use the composition to map [0,1) (via (0,1) to R.
• Oct 24th 2006, 07:40 AM
topsquark
Quote:

Originally Posted by earboth
Hi,

try this one: $\displaystyle f(x)=\frac{\sqrt{x-x^2}}{x-\frac{1}{2}}$

Unfortunately this function maps $\displaystyle [0,1]\mapsto \mathbb{R}$

But may be you find a restriction so you can use your interval.

EB

Actually, the domain of f does not include x = 1/2, so this isn't quite true.

My thought is that this mapping is impossible. The interval [0, infinity) is half closed, whereas the "interval" R is either closed or open, depending on how you look at it. I'm not positive I'm remembering this correctly, but I believe there is a topological theorem that says such functions (mapping from different types of intervals) is not possible, for a continuous function anyway.

Naturally, someone please correct me if I'm wrong. :)

-Dan
• Oct 24th 2006, 08:12 AM
ThePerfectHacker
Quote:

Originally Posted by topsquark

Naturally, someone please correct me if I'm wrong. :)

You are wrong topsquark.

The idea is that $\displaystyle [0,1)$ is the countinuum as does $\displaystyle \mathbb{R}$. To see a prove of this using a variation of the Diagnol argument look here.

Now, by the countinuum hypothesis since $\displaystyle |[0,1)|>\aleph_0$ and $\displaystyle |[0,1)|\leq \mathbb{R}$ we must have,
$\displaystyle |[0,1)|=|\mathbb{R}|$.

The countinuum hypothesis provides us that there does exists a bijection between the two sets. However, whether is is possible to explicity state one, even via Axiom of Choice might not be possible (but I think yes).
• Oct 24th 2006, 08:44 AM
CaptainBlack
Quote:

Originally Posted by srt12
Hi there,

I wonder if you can think of any bijective function that maps [0,1) interval into R?

In case of (0,1) I've done the following: y=tg(pi*x - pi/2), but I have no idea how to deal with [0,1).

Thanks in advance for any help,
Lucas

$\displaystyle \tan(\pi\ x/2)$
• Oct 24th 2006, 09:03 AM
srt12
Quote:

Originally Posted by CaptainBlack
$\displaystyle \tan(\pi\ x/2)$

I'm affraid it returns only positive values for arguments from [0,1).
• Oct 24th 2006, 09:20 AM
srt12
Quote:

Originally Posted by ThePerfectHacker
The countinuum hypothesis provides us that there does exists a bijection between the two sets. However, whether is is possible to explicity state one, even via Axiom of Choice might not be possible (but I think yes).

Thanks a lot, hope it will do that I just show that there exist such a bijection.
And please correct me, if I'm wrong, but continuum hypothesis cannot be neither proved nor disproved, right?
• Oct 24th 2006, 09:29 AM
ThePerfectHacker
Quote:

Originally Posted by srt12
Thanks a lot, hope it will do that I just show that there exist such a bijection.
And please correct me, if I'm wrong, but continuum hypothesis cannot be neither proved nor disproved, right?

Yes it is completely independant from ZFC.
• Oct 24th 2006, 09:37 AM
CaptainBlack
Quote:

Originally Posted by srt12
I'm affraid it returns only positive values for arguments from [0,1).

Opps.

RonL
• Oct 24th 2006, 02:51 PM
Plato
If we can biject (0,1] with R, then certainly we can [0,1).
$\displaystyle (0,1] = \bigcup\limits_{k = 1}^\infty {\left( {\frac{1}{{k + 1}},\frac{1}{k}} \right]}$

Now define a function from (0,1] to (0,1):
$\displaystyle \begin{array}{l} f:(0,1] \to (0,1) \\ x \in (0,1] \Rightarrow \left[ {\exists n} \right]\left( {x \in \left( {\frac{1}{{n + 1}},\frac{1}{n}} \right]} \right) \\ f\left( x \right) = x + 1 - \frac{1}{n} - \frac{1}{{n + 1}} \\ \end{array}$

Show that that is a bijection. Then finish the problem with you function.
• Oct 25th 2006, 12:43 PM
ThePerfectHacker
I do not know what Plato was attempting to do but when I saw that union and I had a revealation.

If you can biject,
$\displaystyle f:A\to B$ and $\displaystyle g:B\to C$ then you can biject,
$\displaystyle A\to C$

Thus, you divide your interval as,
$\displaystyle [0,1),[1,2),[2,3),...$
And you can create a bijection between any adjacent to each other.
That means, the union of these bijective maps (there are $\displaystyle \aleph_o$ many thus the set is well-founded :eek: ) will create a bijection,
$\displaystyle [0,1)\to \mathbb{R}^+$

I am too lazy to work on a procedure but I got the idea from seeing what Plato wrote. (I hope I did not say the same thing).