Alternatives to proving the uncountability of number between 0 and 1

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Discussion Overview

The discussion revolves around alternative methods to prove the uncountability of real numbers between 0 and 1, specifically questioning the sufficiency of Cantor's diagonal argument and exploring simpler approaches.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant suggests a method involving the sequential listing of decimal representations, arguing that one cannot reach certain numbers without first writing others, implying a limitation in listing all reals.
  • Another participant counters that this approach only considers countably many reals with finite decimal representations and asserts that a more sophisticated argument, like the diagonal argument, is necessary.
  • A request for clarification on the claim regarding finite decimal representations is made, indicating a need for further explanation.
  • One participant explains that the set of real numbers in [0,1] with a specific decimal length is finite and argues that the union of these sets remains countable.
  • There are exchanges of frustration regarding the clarity of explanations, with one participant expressing dissatisfaction with perceived insults and another emphasizing the expectation of independent thought in discussions.
  • Concerns are raised about the finiteness of certain sets, with requests for further elaboration on this point.

Areas of Agreement / Disagreement

Participants express disagreement on the sufficiency of the proposed simpler method for proving uncountability, with some advocating for the diagonal argument while others challenge the clarity and validity of the claims made.

Contextual Notes

There are unresolved questions regarding the definitions and properties of finite decimal representations, as well as the implications of countable unions of sets in this context.

robertjford80
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I'm aware of Cantor's diagonal argument but can't you prove the uncountability of reals between 0 and 1 using a simpler method? For instance, take the number .1 sooner or later in order to get a list of the reals between 0 and 1, you're going to have to get to .2 but before you can get to .2 you have to write .11 on your list then before you can get to .12 you have to write .111. In other words if you're obligated to open up a new decimal place then you'll never get to .2
 
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Since there are only countably many reals with finite decimal representations, and since this observation only accounts for these representations, it will not suffice. Something more sophisticated like the diagonal argument is needed.
 
jgens said:
Since there are only countably many reals with finite decimal representations

I need more details as to what you mean.
 
robertjford80 said:
I need more details as to what you mean.

In the future try thinking about the claim for more than three minutes before asking to have it spelled out for you. In any case, notice that the set An of all real numbers in [0,1] whose decimal expansion has length n is in bijection with the set {0,...,9} x ... x {0,...,9} (this is an n-fold product). Since this latter set is finite, it follows that An is finite. Thus the set A of all real numbers in [0,1] with finite decimal representations is given by ∪An and since a countable union of countable sets is again countable the claim follows.
 
jgens said:
In the future try thinking about the claim for more than three minutes before asking to have it spelled out for you.
What a nice person you are. Got a love a man that insults people so easily.

In any case, notice that the set An of all real numbers in [0,1] whose decimal expansion has length n is in bijection with the set {0,...,9} x ... x {0,...,9} (this is an n-fold product).
You're going to have to restate this in informal English.

Since this latter set is finite
I'm not convinced that it's finite. Explain.
 
robertjford80 said:
What a nice person you are. Got a love a man that insults people so easily.

It was not an insult, it was a request. PF is not a place for other people to do the thinking for you. There is an expectation that you think something through before asking.

You're going to have to restate this in informal English.

It would do you good to familiarize yourself with mathspeak, but that point aside I am not sure which part of that sentence is tripping you up, so I need some indication on which parts need clarifying.

I'm not convinced that it's finite. Explain.

Ten minutes is also not a sufficient period of time. Think about the claim some more and if after a couple hours you cannot understand why the set {0,...,9} x ... x {0,...,9} (again an n-fold product here) is finite, then come back and ask.
 
The OP won't be coming back any time soon, so there's no point in keeping this open.
 

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