Surprising rational/irrational formula

  • Context: Undergrad 
  • Thread starter Thread starter uman
  • Start date Start date
  • Tags Tags
    Formula
Click For Summary

Discussion Overview

The discussion revolves around a formula that purportedly determines whether a given number is rational or irrational using limits and elementary functions. Participants explore the implications and utility of this formula, as well as the underlying mathematical reasoning.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant expresses interest in a formula defined as f(x)=\lim_{m\to\infty}[\lim_{n\to\infty}cos^{2n}(m!\pi x)], suggesting it is surprising that such a formula exists.
  • Another participant notes that while the formula is interesting, it may not be practically useful since evaluating the limit would likely require prior knowledge of the rationality of the number.
  • A third participant agrees on the formula's appeal but speculates that its potential applications might detract from its aesthetic value.
  • One participant requests clarification on the theorem being referenced in the discussion.
  • A later reply outlines the reasoning behind the formula, stating that if x is rational, f(x) equals 1, while if x is irrational, f(x) equals 0, based on the behavior of the cosine function and factorials.

Areas of Agreement / Disagreement

Participants generally express interest in the formula, but there is no consensus on its utility or the broader implications of its existence. Some participants find it neat, while others question its practical applications.

Contextual Notes

The discussion does not resolve the practical usefulness of the formula, and assumptions regarding the evaluation of limits and the nature of rational versus irrational numbers remain unexamined.

uman
Messages
348
Reaction score
1
I found this interesting. Maybe a few other people here will too. I would have never thought that there is a formula based only on limits and elementary functions that tells whether a given number is rational or irrational. Anyone else think this is cool?

[tex]f(x)=\lim_{m\to\infty}[\lim_{n\to\infty}cos^{2n}(m!\pi x)][/tex]
 
Physics news on Phys.org
Not that this is useful for any purpose, because to evaluate that limit you'd probably already have to know if the number is rational or irrational... but I still thought it was neat.
 
Sur it's cool.
I'm sure it could be used for something, even though that would detract something from its pure beauty.
 
What exactly is the theorem you are referring to?
 
If x is rational, f(x)=1, because eventually m! is a multiple of the denominator of x, and so m!x is an integer. Then cos^{2n}(m!x pi) = 1, so the inner limit is 1.

If x is irrational, no matter how high m is, m! is not an integer and so cos^2(m!x pi) is less than one, and so the inner limit is equal to zero and hence f(x)=0.
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 9 ·
Replies
9
Views
8K
  • · Replies 85 ·
3
Replies
85
Views
9K
  • · Replies 13 ·
Replies
13
Views
9K
  • · Replies 17 ·
Replies
17
Views
2K
  • · Replies 4 ·
Replies
4
Views
4K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 19 ·
Replies
19
Views
5K
  • · Replies 15 ·
Replies
15
Views
1K