Determining All Continuous Functions s.t.

Click For Summary

Discussion Overview

The discussion centers around determining all continuous functions that satisfy the equation f(x+y)f(x-y) = {f(x)f(y)}^2 for all real numbers x and y. Participants explore methods and approaches to tackle this problem, focusing on theoretical aspects rather than seeking direct solutions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks guidance on methods to approach the problem without wanting direct answers, comparing it to familiar mathematical techniques like factorization.
  • Another participant suggests considering special values of x and y to gain insights into the function's behavior, indicating that this could lead to useful proofs without invoking continuity directly.
  • A different participant mentions Heine's criterion of continuity, proposing it as a relevant concept, though later expresses doubt about its utility in this context.
  • One participant reflects on their own struggles with the problem, admitting to initial confusion and inviting others to share their progress.
  • Another participant asserts that continuous functions are determined by their values on a dense subset, suggesting this might be a key insight.
  • There is a question raised about the existence of non-constant solutions, to which another participant responds affirmatively, hinting at the existence of an uncountable number of solutions.
  • One participant claims to have found a solution but expresses uncertainty about the reasoning that led to it, questioning whether sharing their findings would detract from the exploratory nature of the discussion.
  • A later reply offers to share observations and mistakes made during the problem-solving process, indicating a willingness to contribute to the discussion without providing a complete solution.

Areas of Agreement / Disagreement

Participants exhibit a mix of agreement and disagreement, particularly regarding the methods to approach the problem and the nature of the solutions. While some express confidence in the existence of non-constant solutions, others remain uncertain about their own findings and the reasoning behind them. The discussion does not reach a consensus on the best approach or the completeness of the solutions.

Contextual Notes

Participants acknowledge limitations in their understanding and reasoning, with some expressing confusion about the steps taken to arrive at their conclusions. There is also a recognition of the dependence on specific mathematical properties and the need for further exploration of the problem.

quddusaliquddus
Messages
353
Reaction score
3
I want to determine all continuous functions s.t. for all x, y reals:

f(x+y)f(x-y) = {f(x)f(y)}^2

Now, I want to know where on the web I can learn how I go about doing this, because I don't know what methods to use or what I should be aiming at in manipulating the above.

I don't want an answer - :smile: - I just want to know how to go-about if I want to do this...rather like how I know how to calculate the two roots of an equation, I'd go about factorising for example.

Thank you! :biggrin:
 
Physics news on Phys.org
I realize you don't want solutions just hints, but it's always a fine line. I hope I don't cross it (I certainly won't give you the answer in full because right now I don't know it). But, have you actually considered what the function can be like? In particular, have you considered any special values of x and y? This isn't even using the continuity property (though I am presuming you mean functions from R to R), you'll find there are actually a lot of things you can prove just considering special cases.
 
Right, I think I have figured out the answer, and the thing you need to know about is Heine's criterion of continuity: a function is continuous at x iff for all sequences x_n tending to x, f(x_n) tends to f(x)
 
Cheers. As always - you can trust matt to give you his time. :)
 
I'm just avoiding writing my own maths down, that's all; and this is a less guilty way to 'waste' time (ie pretend it's productive) than playing golf, and counts as a reasonable use of academic resources (free permanently on broadband connection on my desk).

And I haven't solved it as it turns out - I got something the wrong way round.

Let us know how far you've got.
 
Last edited:
Ok, now I've got it, honest, guv. Heine isn't that useful. But knowing that a continuous function is determined by its values on a dense subset is good (and guessing what the answer might be helps as well).
 
matt, are there any non-constant solutions?
 
Yep, there are, at least I'm fairly sure - the constraints mean I can't write them out here for people with more patience than me to check my working. There are, if this isnt' giving the game away too much, an uncountable number of solutions.
 
Last edited:
The thing is I actually have the solution. But I don't understand how I could have come up with the answer. I don't knwo what prompted the person to follow the course of calculations that he did. Do you want to see the whole stuff i got, or would that be spoiling the fun?
 
  • #10
It's your question to do with as you please. If you wish I can give you the observations that I made (including the mistake) that let's you figure out the solutions.
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 2 ·
Replies
2
Views
3K
Replies
21
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 2 ·
Replies
2
Views
4K
  • · Replies 9 ·
Replies
9
Views
3K