1. ## Bijection between Sets(again)

Show that the following pairs of sets S and T are equinumerous by finding a specific bijection between the sets in each pair.

S = [0,1] and T = [0,1)

2. Originally Posted by p00ndawg
Show that the following pairs of sets S and T are equinumerous by finding a specific bijection between the sets in each pair.

S = [0,1] and T = [0,1)

for x<1/2 f(x)=x
for x>=1/2 f(x) = 3/2-x

I guess it works

Sorry - This seems wrong !!

3. Originally Posted by p00ndawg
Show that the following pairs of sets S and T are equinumerous by finding a specific bijection between the sets in each pair.

S = [0,1] and T = [0,1)
Try $f(x) = \left\{ {\begin{array}{rl}
{x,} & {~x \ne \frac{1}
{n},\,\left( {\forall n \in \mathbb{Z}^ + } \right)} \\
{\frac{1}
{{n + 1}},} & {x = \frac{1}
{n},\,\left( {\exists n \in \mathbb{Z}^ + } \right)} \\

\end{array} } \right.$

4. Originally Posted by Plato
Try $f(x) = \left\{ {\begin{array}{rl}
{x,} & {~x \ne \frac{1}
{n},\,\left( {\forall n \in \mathbb{Z}^ + } \right)} \\
{\frac{1}
{{n + 1}},} & {x = \frac{1}
{n},\,\left( {\exists n \in \mathbb{Z}^ + } \right)} \\

\end{array} } \right.$
Plato - Thanks.

Please validate my understanding. You have used a technique similar to the argument used in Hotel Infinity Paradox - To make room available ask every occupant to shift to the next room.

Consider F: [0,1]->[0,1]

If I remove finite points from the range - I can use your argument to construct a bijection. Correct?
Say if range is (0,1), I will map 1/n -> 1/(n+2) and 0 -> 1/2

Correct plz?

5. Originally Posted by Plato
Try $f(x) = \left\{ {\begin{array}{rl}
{x,} & {~x \ne \frac{1}
{n},\,\left( {\forall n \in \mathbb{Z}^ + } \right)} \\
{\frac{1}
{{n + 1}},} & {x = \frac{1}
{n},\,\left( {\exists n \in \mathbb{Z}^ + } \right)} \\

\end{array} } \right.$
hmm, i dont understand this,im not seeing the bijection.

why the 1/(n+1)?

6. Originally Posted by p00ndawg
hmm, i dont understand this,im not seeing the bijection.
You can prove that $f$ is an injection and a surjection for $[0,1]\mapsto [0,1)$.

Have you tried?

7. what do you mean by when x != (1/n)?

im working on the injection on surjection right now, but when x != (1/n) is that for everything in S = [0,1] ?

8. Originally Posted by p00ndawg
what do you mean by when x != (1/n)?

im working on the injection on surjection right now, but when x != (1/n) is that for everything in S = [0,1] ?
Here are some examples,
$f(1)=\frac{1}{2},~f(\frac{1}{2})=\frac{1}{3},~f(\f rac{2}{3})=\frac{2}{3},~f(\frac{\pi}{4})=\frac{\pi }{4}$

9. Originally Posted by Plato
Here are some examples,
$f(1)=\frac{1}{2},~f(\frac{1}{2})=\frac{1}{3},~f(\f rac{2}{3})=\frac{2}{3},~f(\frac{\pi}{4})=\frac{\pi }{4}$
Okay! thank you! I think im finally getting it.

I just have one more question, when im looking at these two sets how would I know to
separate it into a function like that?

is it just practice? or is there anything from that set that is setting off some alarms in your head?

thanks a bunch!