Uniform Convergence of Fn(x)=nx(1-x^2)^n on [0,1]?

In summary, the conversation discusses whether the function Fn(x)= nx(1 - x^2)^n converges uniformly on the interval [0,1]. While the initial instinct is that it does converge uniformly to 0, the speaker is unable to prove this using the definition. They then mention the use of the sup norm of f_n and suggest considering turning points and points of inflection. It is determined that the sup norm of f_n is 1, which means that the function does not converge uniformly on [0,1]. The importance of considering the limit and the need for pointwise convergence is also mentioned in the conversation.
  • #1
math-chick_41
34
0
does Fn(x)= nx(1 - x^2)^n converge uniformly on [0,1]?
my first instinct was yes it converges uniformly to 0
but I can't seem to show that using the definition.

i get |nx(1 - x^2)^n|<=|nx|<=n for x in [0,1]

any tip or hint would be helpful
thanks
 
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  • #2
What is the sup norm of f_n? That should give you the answer. Hint, it might help to remember basic analysis like turning points and points of infelxion...
 
  • #3
for n large and x really small, namely x=1/n
i get that the sup{|f_n(x)|:x in [0,1]} = 1
so then it doesn't converge uniformly.
is that right?
 
  • #4
That'll be right as long as you believe that the limit, if it were to exist, were the zero function, or something like it. After all, you need to have the notional idea of what it might converge to if it were to converge (and then to show that it is not uniform convergence). So, show it converges pointwise, and then show it is not uniform (if it were uniform then it would certainly be pointwise).

I don't agree that is important we say *for n really large* and *x really small* the the sup is something, rather than for ANY n the sup is..., after all the sup norm ought to be at least something independent of the n.
 
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1. What is uniform convergence?

Uniform convergence refers to a type of convergence of a sequence of functions where the convergence is uniform across the entire domain of the function. This means that the rate of convergence is the same at every point in the domain, rather than just converging at certain points.

2. How is uniform convergence different from pointwise convergence?

Pointwise convergence refers to a type of convergence of a sequence of functions where the convergence occurs at each individual point in the domain. This means that the rate of convergence can vary at different points. In contrast, uniform convergence ensures that the rate of convergence is consistent across the entire domain.

3. What is the definition of uniform convergence?

A sequence of functions f_n(x) is said to converge uniformly to a function f(x) if for any positive number ε, there exists a natural number N such that for all x in the domain, when n is greater than or equal to N, the difference between f_n(x) and f(x) is less than ε.

4. Why is uniform convergence important?

Uniform convergence is important because it guarantees that the limit function is continuous. This is useful in many areas of mathematics and science, as continuity is a necessary condition for many important theorems and applications.

5. How is the uniform convergence of Fn(x)=nx(1-x^2)^n on [0,1] determined?

To determine the uniform convergence of Fn(x)=nx(1-x^2)^n on [0,1], we can use the Weierstrass M-test. This test states that if there exists a sequence of positive numbers M_n such that for all x in the domain, when n is greater than or equal to N, the absolute value of f_n(x) is less than or equal to M_n, and the series M_n converges, then the sequence of functions f_n(x) converges uniformly.

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