Is the Set M Closed in the Space X?

In summary, the conversation is about determining whether the set M, which is a subset of X, is closed in X. The set M is defined as a set of continuous functions on the interval [0,1] that have an integral of 2 over that interval and have a value of 0 at the endpoint 1. The conditions for M to be closed are checked and the discussion touches on the type of convergence and the justification for the commuting of the integral and limit.
  • #1
iris_m
8
0

Homework Statement


Let [tex]X=(C([0,1]), || . ||_1 )[/tex], where [tex]||f||_1=\int_{0}^{1}|f(t)|dt[/tex].
Let [tex]M=\{f \in C([0,1]) : \int_{0}^{1}f(t)dt=2, f(1)=0\}[/tex].
Is M closed in X?

The Attempt at a Solution



I've tried the following:
Let [tex]f_n[/tex] be a sequence in M such that [tex]f_n \rightarrow f[/tex].
I'm checking whether [tex]f \in M[/tex], because that would prove that M is closed (if a set contains all the limits of its convergent sequences, it is closed).
There are obviously two conditions to check.
The first one:
[tex]\int_{0}^{1}f(t)dt=\int_{0}^{1}limf_n(t)dt=lim\int_{0}^{1}f_n(t)dt=lim 2=2[/tex].
Now I have to check the second one, that is, is [tex]f(1)=0[/tex], and I don't know how.

Any help is much appreciated.
 
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  • #2
If [itex]f_n(1)= 0[/itex] and [itex]f_n\rightarrow f[/itex], then... (What kind of convergence are you talking about? Pointwise? Uniform? In the norm?)

And is your justification for saying
[tex]\int_0^1 limf_n(t)dt= lim\int_0^1 f_n(t)dt[/tex]?
 
  • #3
HallsofIvy said:
If [itex]f_n(1)= 0[/itex] and [itex]f_n\rightarrow f[/itex], then... (What kind of convergence are you talking about? Pointwise? Uniform? In the norm?)

In the norm.

And is your justification for saying
[tex]\int_0^1 limf_n(t)dt= lim\int_0^1 f_n(t)dt[/tex]?

My justification would be that f_n are continuous functions, so integral and limit commute.
 

1. What is a closed set in a normed space?

A closed set in a normed space is a set that contains all of its limit points. In other words, any sequence of points in the set that converges to a point outside the set is not considered a limit point. This concept is important in topology and analysis, as it helps define continuity and convergence.

2. How is a closed set different from an open set in a normed space?

An open set in a normed space is a set that does not contain any of its boundary points, while a closed set contains all of its boundary points. In other words, a set is closed if it includes its endpoints, while an open set does not include its endpoints. This distinction is important in the study of continuity and convergence.

3. What is the importance of closed sets in functional analysis?

Closed sets are important in functional analysis because they help define the properties of topological vector spaces and help determine the behavior of sequences and series within those spaces. They are also essential in understanding the concept of completeness, which is a key property of normed spaces.

4. Can a set be both open and closed in a normed space?

In general, no. A set can only be both open and closed in a normed space if it is the empty set or the entire space. This is because the definitions of open and closed sets are complementary, so a set cannot contain all of its boundary points and none of its boundary points at the same time.

5. How do closed sets relate to the concept of convergence in a normed space?

Closed sets play an important role in determining convergence in a normed space. In particular, a sequence of points in a normed space will converge to a point if and only if that point is contained in every closed set containing the sequence. This is known as the closed set convergence criterion and is a powerful tool in functional analysis.

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