Proving a Continuous Function has a Fixed Point

In summary, the question is asking if the function g(x)=f(x)-x ever crosses the X-axis, with a hint that g can be both positive and negative. The minimum value of g can be 0 if f(x) equals 1 or 1/3 + 2x/3. It is also mentioned that g has a fixed point, but it is unknown how to show that g has a root for all functions f. There is a correction that 'positive' and 'negative' are defined as \geq 0 and \leq 0 instead of > 0 and <0.
  • #1
JellyFox
2
0
This is a question from the exam for the calculus class I took last semester:

1z5m1zp.jpg


It looks like it might be able to be done with squeeze theorem, but I can't work it out. Please help me with this, before I descend into madness.
 
Physics news on Phys.org
  • #2
Consider the function g(x)=f(x)-x. Does this ever cross the X-axis? (HINT: the function g is sometimes positive and sometimes negative)
 
  • #3
micromass said:
Consider the function g(x)=f(x)-x. Does this ever cross the X-axis? (HINT: the function g is sometimes positive and sometimes negative)

But it isn't necessarily negative. If f(x) = 1, then the minimum value of g is is 0, or if f(x) = 1/3 + 2x/3 then the minimum value of g is 0 again. And they show that there's a fixed point, but I don't know how to show that g has a root for all functions f. Thanks.
 
  • #4
JellyFox said:
But it isn't necessarily negative. If f(x) = 1, then the minimum value of g is is 0, or if f(x) = 1/3 + 2x/3 then the minimum value of g is 0 again.
Correct, but g necessarily takes the value 0 somewhere, right? That's all you need.

(Or we could say that micromass's hint is correct, provided that 'positive' resp. 'negative' is defined as [itex]\geq 0[/itex] resp. [itex]\leq 0[/itex], instead of [itex]> 0[/itex] resp. [itex]<0[/itex] :) )
 
  • #5
Yes, I'm sorry, a function is positive for me if it is [tex]\geq 0 [/tex]. Sorry for the confusion...
 

FAQ: Proving a Continuous Function has a Fixed Point

1. What is a continuous function?

A continuous function is a mathematical function that maintains its value as the input values change. This means that there are no abrupt changes or breaks in the graph of the function.

2. What is a fixed point?

A fixed point of a function is a value that remains unchanged when the function is applied to it. In other words, if the input and output of a function are the same, then that value is a fixed point.

3. How do you prove that a continuous function has a fixed point?

To prove that a continuous function has a fixed point, you can use the Intermediate Value Theorem. This theorem states that if a continuous function has different signs at the endpoints of an interval, then it must have at least one root (or fixed point) within that interval.

4. Are there any other methods to prove a continuous function has a fixed point?

Yes, there are other methods to prove a continuous function has a fixed point. One method is the Brouwer Fixed Point Theorem, which states that any continuous function from a closed and bounded set to itself must have at least one fixed point.

5. Can a non-continuous function have a fixed point?

No, a non-continuous function cannot have a fixed point. This is because a fixed point is a value that remains unchanged as the function is applied to it, and a non-continuous function will have abrupt changes or breaks in its graph, making it impossible for any input to remain unchanged.

Similar threads

Back
Top