Generating Real Numbers: Is It Possible?

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Discussion Overview

The discussion revolves around the possibility of generating real numbers through formulas, infinite series, or continued fractions. Participants explore the implications of countability and uncountability in relation to the sets of natural numbers and real numbers, as well as the limitations of expressing real numbers through finite expressions.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether a formula or series could generate all real numbers, noting the challenge of changing a formula to produce different reals without implying a one-to-one correspondence with naturals.
  • Another participant clarifies that if real numbers are allowed as variables, a simple function like "f(x) = x" could represent all real numbers, but this raises questions about the nature of the variables used.
  • A participant suggests that while they can express e^x as a series, they doubt the feasibility of writing a series for every real number due to the uncountable nature of reals.
  • Another participant elaborates that writing e^x as a series does not allow for the entire series to be expressed, emphasizing the countable nature of finite expressions compared to the uncountable set of real numbers.
  • It is noted that infinite expressions could represent all reals, as any real number can be expressed through its decimal expansion.
  • A participant points out that evaluating e^x for irrational x is necessary to obtain all real numbers as values.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of generating real numbers through formulas or series. There is no consensus on whether it is possible to represent all real numbers in this manner, and the discussion remains unresolved.

Contextual Notes

The discussion highlights limitations related to countability and the nature of expressions, particularly the distinction between finite and infinite expressions in relation to the set of real numbers.

cragar
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Could it be possible to come up with a formula or infinite series or continued fraction to generate the real numbers? I might have to change something in my formula to generate another real. I couldn't just change one of my numbers in the formula because then i would be saying there is a one-to-one correspondence between the naturals and the reals and there isn't. But what if I used irrational numbers to change my formula or something like that.
And i don't think we could generate the reals in order because there is no next real on the number line, maybe its not possible, what do you guys think.
 
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What do you mean by a "formula"? A formula in terms of what kind of "variable"? integers? rational numbers? Neither of those is possible because the set of all rational numbers (and so the set of all integers) are both countable while the set of all real numbers is uncountable. If you allow real numbers as variables, then the simplest such formula is "f(x)= x". That will give every real numbers as a function of some real number x!
 
okay what i mean is, and this probabaly won't work because their uncountable . But i can write e^x as a series and then evaulate e with this series, so why couldn't I write a series for every real number.
 
cragar said:
okay what i mean is, and this probabaly won't work because their uncountable . But i can write e^x as a series and then evaulate e with this series, so why couldn't I write a series for every real number.

Well you really can't write e^x as a series. What I mean is, you can't write down the entire series. What we do is write out an expression for the general term of the series; or we write down a finite number of terms of the series and end with dot dot dot. In either case we are writing a finite-length expression.

There are only countably many finite-length expressions from a finite or countable alphabet (you should prove this for yourself, it will give you a good feel for the countable nature of the set of finite expressions).

But there are uncountably many reals. So most of the reals can not be described by a finite expression.

If we allowed infinite expressions, then any real could be expressed by some infinite expression. For example we could write down the entire decimal expansion of each real. So if we allow for expressions of infinite length, there are indeed uncountably many of those.
 
Writing [itex]e^x[/itex] as a series still requires that you evaluate it for irrational x to get all real numbers as values.
 

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