- #1
MaxManus
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Homework Statement
(X,d) is a metric space
B(x) is the set of all bounded functions on X
show that
f = {g [tex]\in[/tex] B(X) g is continious}
is a closed subset
The Attempt at a Solution
A hint of how to start?
Definition of a closed subset?MaxManus said:Homework Statement
(X,d) is a metric space
B(x) is the set of all bounded functions on X
show that
f = {g [tex]\in[/tex] B(X) g is continious}
is a closed subset
The Attempt at a Solution
A hint of how to start?
MaxManus said:Thanks
A subset of a metric space is closed if it contains all its boundary
points.
But I don't know how to apply it.
MaxManus said:Thanks, but I don't understand what you are asking for.
If f_n has limit f then
abs(d(f_n, -f)) <= epsilon
for large enough n
Is this something I should use?
MaxManus said:show that d(f,a) <= M, where M <= infinity
d(f,a) <= d(f,f_n) + f(f_n,a) <= epsilon + M = M
Is this correct?
where "a" is in B(X)
MaxManus said:Thanks, I still don't understand.
I'm to show that:
|f(x)| < M
when:
|f_n| < M, for all n
|f_n -f| < epsilon, for n > N
But how?
MaxManus said:|f_n -f| < epsilon
-epsilon<f_n - f < epsilon
-epsilon<M-f <epsilon
-M-epsilon <-f < epsilon - M
M + epsilon > f > M-epsilon
right?
But how does this show that f is closed?
MaxManus said:But its right now now?:
|f_n -f| < epsilon
-epsilon<f_n - f < epsilon
-f_n-epsilon <-f < epsilon - f_
f_n + epsilon > f > f_n - epsilon
M +epsilon > f > -M-epsilon
MaxManus said:But if
f_n + epsilon > f > f_n - epsilon
and:
-M<f_n<M
why not
M +epsilon > f > -M-epsilon
f_n can't be bigger than M or less than -M
MaxManus said:Don't think I understand the problem;
f = {g [tex] \in [/tex] B(X) g is continuous}
Doesn't that mean that f is continuous?
MaxManus said:If f is a function X -> Y, between two metric spaces.
f is continuous if for each epsilon > 0 there is a delta > 0 such that [tex] d_Y(f(x),f(y)) < \epsilon [/tex] for all x any y in X such that [tex] d_X(x,y) < \delta [/tex]
But is this possible to use when you don't know the function?
MaxManus said:|f_n(x)-f_n(y)|<epsilon for all d(x,y)<delta
|f_n -f| < epsilon2 for n > N
Triangle inequality
d(f(x),f(y)) <= d(f(x),f_n(x)) + d(f_n(x),f_n(y)) + d(f_n(y),f(y))< 2*epsilon2 + epsilon = epsilon3.
MaxManus said:But how do we go from here to knowing that F contains all its boundary points?
A subset of a metric space is closed if it contains all its boundary
points
MaxManus said:You've used that the f_n are continuous and bounded to show f is continuous and bounded. Doesn't that mean f is in F?
I don't see it. But I know that if f is in F then it is closed
MaxManus said:I ment:
If {x_n} is a convergent sequence of elements in F, then the limit a =
lim x_n always belongs to F. = F is closed. I thought you used this rule, but no?
A set is considered closed if it contains all of its limit points. In other words, if a sequence of points within the set converges, the limit point must also be within the set.
To prove that a set is closed, you can use the definition of a closed set and show that it contains all of its limit points. This can be done through various methods such as using the closure of a set or the sequential definition of a limit point.
Yes, a set can be both open and closed. This is known as a clopen set and it is possible in certain topologies, such as the discrete topology where all subsets are both open and closed.
In general, a function is continuous if and only if the preimage of a closed set is also a closed set. This means that if a set is closed, the function will preserve its closedness. Therefore, proving that a set is closed can be useful in showing continuity of a function.
One example of a closed set is the set of all real numbers greater than or equal to 0. This set includes all of its limit points, such as 0, and therefore is considered closed.