Hopefully easy question about sups of continuous functions

In summary, the question is whether for a continuous linear functional on a normed space, the supremum over all x with norm less than 1 is equal to the supremum over all x with norm equal to 1. There is some confusion about the definition of "functional" and whether it necessarily means "linear functional". The statement seems to hold for R but it is uncertain for an arbitrary normed space.
  • #1
AxiomOfChoice
533
1
If f is a continuous functional on a normed space, do you have

[tex]
\sup_{\|x\| < 1} |f(x)| = \sup_{\|x\| = 1} |f(x)|
[/tex]

If so, why? If not, can someone provide a counterexample?
 
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  • #2
That's an interesting question. A normed space must be a vector space right? But does a vector space have to be connected?
 
  • #3
try it on R. can you think of a function which gets larger somewhere inside the unit interval than it is at 1, -1? maybe you meant linear. or maybe functional means linear. then it seems true.
 
  • #4
mathwonk said:
try it on R. can you think of a function which gets larger somewhere inside the unit interval than it is at 1, -1? maybe you meant linear. or maybe functional means linear. then it seems true.

Yeah, I did mean to put "linear functional" above...I guess I was under the impression that "functional" implies "linear functional", though I'm not at all sure that's the case.

And yeah, it clearly seems true on R, but for an arbitrary normed space, I'm not so sure...
 

What is a sup of a continuous function?

A sup of a continuous function is the smallest upper bound of the function's values. In other words, it is the largest value that the function can take on within a given interval.

How is a sup of a continuous function calculated?

To calculate the sup of a continuous function, you would need to find the maximum value of the function within the given interval. This can be done by finding the critical points of the function and evaluating the function at those points, as well as evaluating the function at the endpoints of the interval.

Can the sup of a continuous function be infinite?

Yes, the sup of a continuous function can be infinite. This would occur when the function has no upper bound within the given interval, meaning that the function's values increase without limit.

What is the relationship between the sup of a continuous function and its graph?

The sup of a continuous function is the highest point on the function's graph within the given interval. It is also the point where the graph of the function begins to decrease, as the function's values cannot exceed the sup.

Why is the sup of a continuous function important?

The sup of a continuous function is important because it helps us understand the behavior of the function within a given interval. It can also be used to determine if the function has a maximum value within the interval, as well as to evaluate limits and continuity of the function.

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