Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Let $X$ denote the two point set $\{0,1\}$ and let $X_j=\{0,1\}\forall j=1,2\dots$ let $Y=\Pi_{j=1}^{\infty}X_j$, I need to determine whether each of the following are true or false:

  1. $Y$ is countable

  2. $|Y|$=|[0,1]|

  3. $\bigcup_{n=1}^{\infty}\Pi_{j=1}^{n}X_j$ is uncountable

  4. $Y$ is uncountable.

I guess $Y$ is uncountable (4), but I can not prove it.

share|cite|improve this question
Do you have any ideas about any of the four? – Brian M. Scott Jun 13 '12 at 10:42
We have to show any finction $g:\mathbb{N}\rightarrow Y$ is not surjective. – Un Chien Andalou Jun 13 '12 at 10:45
Is #3 is a typo? You probably meant $n$ for the second infinity? – rschwieb Jun 13 '12 at 11:02
Yes, I guess so. – Un Chien Andalou Jun 13 '12 at 11:04
@mex If you start probing why you feel justified in guessing your guess, more of your guesses will turn into solutions! If you include more of your thoughts about your guess, we can help you do this. – rschwieb Jun 13 '12 at 11:53
up vote 5 down vote accepted


  1. On a meta-level, if the first is true then the other three are trivially false.
  2. Every real in $[0,1]$ has a binary decimal expansion $0.b_1b_2b_3\ldots$
share|cite|improve this answer
Note that the corrected version of (3) is no longer incompatible with (1) and is in fact false. – Brian M. Scott Jun 13 '12 at 23:31

Every $a \in Y$ is an infinite series of zeros and ones. Think of any $b \in P(\mathbb N)$ and try to find the connection between this two.

share|cite|improve this answer

Proof that 3 is false.

List the elements of the union of finite cartesian products of {0,1} by increasing $n$ in layers.

Within layer list them by increasing size of the corresponding binary number:

0, 1 $(n=1)$

00, 01, 10, 11 $(n=2)$

000, 001, 010, 011, 100, 101, 110, 111 $(n=3)$

0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111, 1000, 1001, 1010, 1011, 1100, 1101, 1110, 1111 $(n=4)$


Now label the list using the natural numbers, first by layer, then moving to the next layer.

The elements in the $n$-th layer will be labeled by $2$$n$-$1$ through $2$$n+1$-$2$.

This provides a bijection between N and all elements of the union of finite cartesian products of {0,1}, so the union of finite cartesian products of {0,1} is countable.

And I think it is done without the assistance of my favourite set theoretical bogeyman, Axiom of Choice.

P.S. Regarding point 1 in the original question, is there any relation between the infinite countable product of {0,1} and w1 (the first uncountable ordinal)?

share|cite|improve this answer

Your Answer


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

Not the answer you're looking for? Browse other questions tagged or ask your own question.