docnet said:
Is the existence of such a convergent sequence guaranteed by ##f## being continuous? It seems a bit unnatural to define the sequence in the image in terms of the sequence in the domain.
Do you mean a sequence converging to the supremum or a convergent subsequence that exists by Bolzano-Weierstraß?
Neither is because of continuity. The former is because of the definition of a supremum. We either have a maximum and a constant sequence, or we can find a value closer to that with which we started and so forth. The latter is as in the old joke about how a mathematician catches a hare: he divides the terrain into half and looks in which half the hare is. Then he divides that part into half and looks where the hare is, and so on. The crucial point here is the boundness of the unit ball.
docnet said:
The closedness of the domain seems like a crucial part of the proof because ##\lim_{n\to \infty}d_{n_k}=d## being within the ball allows us to argue that ##f(d)=M## is in the image.
Indeed. Without it, like in an open ball ##\{x\, : \,\|x\|<1\}## we could find that ##\|d\|=1## and fail to find a pre-image of ##M.##
docnet said:
At this stage, we haven't assumed that this ##d## makes ##f(d)=1##, right? The Bolzano-Weierstraß theorem just seems to say there is a convergent subsequence in the domain, and doesn't seem to specify what the subsequence converges to. If so, I have no issues with it.
Right. But as it is a convergent sequence ##(d_{n_k})## within a convergent sequence ##(d_n)## it cannot have two distinct limits. The norms of the sequence members are irrelevant, except that they are all within the domain of ##f## since we are searching for a limit ##d## such that ##f(d)=M.## The norm of ##d## is also irrelevant as the examples show where ##d## is placed at the origin of the ball.
docnet said:
Proofs like these are surprising to me because it's all so unintuitive but it works.
Look up the German Wikipedia pages (and eventually use Chrome to translate the page.) At least the proof of Bolzano-Weierstraß has some nice images that describe the situation:
https://de.wikipedia.org/wiki/Satz_von_Bolzano-Weierstraß#Beweisskizze
We need the boundness to avoid that ##d## slips into the infinities, and we need closeness to avoid that ##d## escapes into loopholes like ##\mathbb{R}\backslash \mathbb{Q}## or ##[-1,1] \backslash (-1,1).##
docnet said:
Is this a well-known result?
Yes, it has a name, with or without mentioning Weierstraß. I guess that results with names can generally be considered well-known. But this result is important because it provides a solution to the problem of finding an extreme value. This is an important part of many other proofs. And the reason why the joke with the hare works. And it guarantees that a numerical solution by an algorithm converges.
docnet said:
By the way, have you thought about getting on TRT? A lot of 20+ year old guys seem to rave about its positive effects on concentration.
It's far better today. You could have complained that I posted although I knew that I had a weak day, but I somehow got sucked in. Mathematics is a never-ending sequence of the constantly posed question "why". One has to convince oneself over and over again that the next step in a chain of conclusions is correct. Sometimes you simply get lost somewhere in a trapdoor.
But, yes, age plays a significant role. Here are two sad, however, prominent examples where age was the problem:
https://www.physicsforums.com/threads/has-the-riemann-hypothesis-been-proven.955832/
https://www.physicsforums.com/threa...-the-history-of-physics.1059261/#post-7005132
Not that I would compare myself with either of the two, never ever, but it shows that it could be worse.