# Thread: Infinite sets and Countable sets

1. ## Infinite sets and Countable sets

Hi everybody,

I need to prove the following sets are infinite sets by finding a countable in everyone of them:
a) $\mathbb{Q}\backslash\mathbb{Z}$
b) $\mathbb{R}\backslash\mathbb{Q}$
c) $\{0,1\}^\mathbb{N}$
d) $\mathbb{N}^{\{0,1\}}$

How can I do this?
I was wondering if the following would be correct:
for example in (a): $\mathbb{A}\subset\mathbb{Q}\backslash\mathbb{Z}$,
where $\mathbb{A}= \{ x | x = \frac{1}{n}, & \forall n\in \mathbb{N} \}$.
Thanks in advance for the help.

2. ## Re: Infinite sets and Countable sets

Originally Posted by jfk
I was wondering if the following would be correct:
for example in (a): $\mathbb{A}\subset\mathbb{Q}\backslash\mathbb{Z}$,
where $\mathbb{A}= \{ x | x = \frac{1}{n}, & \forall n\in \mathbb{N} \}$.
This is almost correct except that 1 ∈ A and, depending on the definition of natural numbers, 0 may be a natural number. Also, it is more correct to write this version of A as $\{x\mid\exists n\in\mathbb{N}\;x=1/n\}$ or $\{1/n\mid n\in\mathbb{N}\}$.

Why don't you try other parts and post the results for verification?

3. ## Re: Infinite sets and Countable sets

Thanks emakarov I just realized that 1 and 0 are problematic in my definition of A.
I'll think about (b)(c)(d) and I'll post them later...

4. ## Re: Infinite sets and Countable sets

For (b) I'm not sure about that one. If I take out all the rationals from the real numbers, then I'll have only the transcendentals in that remaining set and therefore it cannot be counted... Is that right?

5. ## Re: Infinite sets and Countable sets

Originally Posted by jfk
to prove the following sets are infinite sets by finding a countabl(y infinite subset) in everyone of them:
b) $\mathbb{R}\backslash\mathbb{Q}$.
Have you thought about $\left\{ {\frac{\pi }{{{2^n}}}:~n \in \mathbb{N}} \right\}~?$

6. ## Re: Infinite sets and Countable sets

ouch! Nope I forgot about that. Thanks

7. ## Re: Infinite sets and Countable sets

@jfk Just a correction to something you said:

Not every irrational number is transcendental. For example square roots of non-perfect squares are irrational, but not transcendental.

Hint for (c): functions from the naturals to {0,1} are essentially just countable sequences of 0's and 1's. Can you write down infinitely many such sequences? There are many ways to do this.

Hint for (d): elements of this set are essentially ordered pairs of natural numbers. Can you write down infinitely many such pairs?

8. ## Re: Infinite sets and Countable sets

Originally Posted by DrSteve
Not every irrational number is transcendental. For example square roots of non-perfect squares are irrational, but not transcendental.
10x DrSteve for the correction. Though I'm not sure I understood your example. I'm pretty confused about the relation (e.g: who contains who) between Irrational, Algebraic and Transcendental numbers, I would apreciate it very much if some one can help me make some order with those concepts.

10. ## Re: Infinite sets and Countable sets

I'm sorry I still don't understand (c) and (d), what's the meaning of a set to the power of another set?

11. ## Re: Infinite sets and Countable sets

Originally Posted by jfk
I'm sorry I still don't understand (c) and (d), what's the meaning of a set to the power of another set?
${\left\{ {0,1} \right\}^\mathbb{N}}$ is the set of all functions mapping $\mathbb{N}\to\{0,1\}$.
Think characteristic functions

${\mathbb{N}^{\left\{ {0,1} \right\}}}$ is the reverse of that.

12. ## Re: Infinite sets and Countable sets

Ok then,

(c) could be $\left\{f|f(x) = \left\{ \begin{array}{rcl}1 & \mbox{for x is Even} \\ 0 & \mbox{for x is Odd}\end{array}\right.\right\}$ ?

13. ## Re: Infinite sets and Countable sets

Originally Posted by jfk
Ok then,

(c) could be $\left\{f|f(x) = \left\{ \begin{array}{rcl}1 & \mbox{for x is Even} \\ 0 & \mbox{for x is Odd}\end{array}\right.\right\}$ ?
That would be a single function in the set, not a countabe collection of them.

14. ## Re: Infinite sets and Countable sets

What about $\{f\mapsto$$f(1),f(2),f(3),...$$\}$???

15. ## Re: Infinite sets and Countable sets

Originally Posted by jfk
What about $\{f\mapsto$$f(1),f(2),f(3),...$$\}$???
that's not good, either. what about a function f for which f(k) = 1, and f(n) = 0, if n ≠ k? how many of THOSE functions are there?

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