Proving Existence of Fixed Points in Continuous Sets

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Homework Help Overview

The problem involves proving the existence of at least one fixed point for a continuous function f mapping from the interval [a,b] to itself. A fixed point is defined as a value x within the interval such that f(x) = x.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to understand the concept of a fixed point and considers using the Intermediate Value Theorem (IVT) but expresses uncertainty about the definitions involved. Some participants suggest applying the IVT to the function f(x) - x and discuss the implications of the function's mapping within the interval.

Discussion Status

The discussion is ongoing, with participants providing hints and clarifications regarding the application of the IVT. There is a collaborative atmosphere as participants share insights without fully detailing the solution process.

Contextual Notes

There is a mention of the need to understand the definitions and implications of the fixed point and the continuity of the function, which may be contributing to the original poster's confusion.

angelpsymon
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Homework Statement


Suppose f:[a,b] [tex]\rightarrow[/tex] [a,b] is continuous. Prove that there is at least one fixed point in [a,b] - that is, x such that f(x) = x.


Homework Equations





The Attempt at a Solution


I was going to try something with the IVT, but then I realized I wasn't sure what they meant by a fixed point much less how to solve this problem. Any help would be appretiated.
 
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Hi Angelpsymon,

It says what a fixed point is in the problem statement: "x such that f(x) = x." You are absolutely correct in thinking to apply the intermediate value theorem. Hint: since f maps into [a,b], we must have that [tex]f(a)\geq a[/tex] and [tex]f(b)\leq b[/tex].
 
As Unco suggested, maybe without spelling it out completely, apply the IVT to f(x)-x.
 
Dick said:
As Unco suggested, maybe without spelling it out completely
Apologies, Dick, I certainly didn't mean to do so.
 
You don't HAVE to spell it out completely. Hints are enough. I apologize if I spoiled your hint and made it too obvious. I was just saying how to apply the IVT.
 
Alright, I think that I got it now. Thanks a lot guys.
 

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