# Search results

1. ### Challenge Math Challenge - August 2020

Yes, I was talking about ##Y##. But both the approaches given use a shrinking family of neighborhoods, which seems to suggest at least that we deal with first countable spaces?
2. ### Challenge Math Challenge - August 2020

Ah nice, a more topological approach. I didn't write out the details of your post but this seems to suggest that the statement is more generally true than in metric spaces. I would be interested in knowing what minimal assumptions are such that this holds. I guess the Hausdorffness condition and...
3. ### B Why is a one-variable linear equation called linear?

@etotheipi is not wrong. Linearity refers (in the context of abstract algebra) to both the preservation of the scalar multiplication and the addition. A linear map is a map that is both homogeneous and additive.
4. ### Challenge Math Challenge - August 2020

Close enough. The reverse inequality is not quite the definition of continuity but you need a routine triangle inequality argument to get there, like you did in the other inclusion. But I'm sure you could have filled up the little gap. Well done! I think your solution is the the shortest possible.
5. ### Challenge Math Challenge - August 2020

Indeed for this first set equality you needed something like injectivity which is not given. Can you edit or make a new attempt at the question so everything reads smoothly? Otherwise everything is shattered through multiple posts making it very hard to read for others (and me when I go through...
6. ### Challenge Math Challenge - August 2020

I think there is a problem with your intersection, at least semantically. You have an intersection ##\bigcap_{n \in \Bbb{N}} A_n## with ##A_n \subseteq \mathcal{P}(X)##. Thus semantically your intersection is a subset of ##\mathcal{P}(X)##. However, the left hand side is ##\{x\}## which is a...
7. ### Matrix concept Questions (invertibility, det, linear dependence, span)

Really any book on linear algebra has proofs of these facts. Have you consulted one of those? I like Axler's book on linear algebra :)
8. ### I The Set of Borel Sets ... Axler Pages 28-29 ... ...

Question 1 and question 2 are dual (use complements), so I will only answer question 1. Q1: Axler claims that the set ##\mathcal{B}:=\{\bigcap \epsilon\mid \epsilon \mathrm{\ countable \ collection \ of \ opens}\}## is not equal to all Borel sets. His argument is: showing that there exists a...
9. ### A The meaningfulness item on math probability

I think this is getting more at statistics than probability theory.
10. ### I Inequality from a continuity exercise

I'm not sure what you are after: $$\frac{|f(x)-f(x_0)|}{5} = \frac{5|x-x_0|}{5}= |x-x_0|$$ and the inequality you are looking for so hard is always true (it is even equality!) regardless of the fact that ##|x-x_0|## is small. I agree with @PeroK that the author was probably sloppy and yes it...
11. ### I Inequality from a continuity exercise

$$|x-x_0| <\delta \implies |y-y_0| = |f(x)-f(x_0)| = |5x+3-(5x_0+3)| = 5|x-x_0|< 5\delta$$ This suggest taking ##\delta = \epsilon/5## in an ##\epsilon-\delta## proof.
12. ### I Inequality from a continuity exercise

It is unclear from your post what ##y_0## is. I'm guessing you want to show that ##f## is continuous at ##x_0## and ##f(x_0) = y_0##?
13. ### I Smallest Sigma Algebra ... Axler, Example 2.28 ...

I'm very sorry. I made a mistake in my definition of ##\mathcal{S}## in post #2 (now fixed), which is probably why your attempt doesn't work because my definition of ##\mathcal{S}## contained a mistake. I guess that happens when I give hints without writing down anything on paper ;) But as you...
14. ### Challenge Math Challenge - August 2020

It got lost in the sea of other posts haha. I will have a look.
15. ### Challenge Math Challenge - August 2020

Your first solution + the clarification in post #54 solves the question (I didn't look at the second one though)! Well done! I guess I must come up with less routine exercises since you seem to solve all of them ;)
16. ### Challenge Math Challenge - August 2020

This is much more readable to me than your previous post. Your approach has all the right ideas. Especially the line "The only continuous functions of the form ##2\pi n(\theta)## are constant functions." is the key to an elementary approach that does not use black magic. I consider this...
17. ### I Smallest Sigma Algebra ... Axler, Example 2.28 ...

You should show an attempt. Can you at least show that ##\mathcal{S}:=\{E \subseteq X: E \mathrm{\ countable \ or \ E^c \ countable}\}## is a ##\sigma##-algebra containing ##\mathcal{A}##? Why is it the smallest?
18. ### I Sigma Algebras ... Axler, Page 26 ...

Axler does not say that. The collection of all subsets is a ##\sigma##-algebra (trivially). Axler says that we cannot define Lebesgue-measure on this ##\sigma##-algebra and that's why we define Lebesgue measure on Borel sets.
19. ### Other How is one supposed to study advanced subjects when textbooks don't even have answers to the exercises?

But if the students can solve it correctly, so should the teacher! I'm speaking high school here. At the university level I can imagine this happening.
20. ### Other How is one supposed to study advanced subjects when textbooks don't even have answers to the exercises?

Yes, that's the right course of action but if you can't solve a problem without a book, why do you expect students can solve it without the book?
21. ### Other How is one supposed to study advanced subjects when textbooks don't even have answers to the exercises?

Sounds like a really bad high school teacher.
22. ### Challenge Math Challenge - August 2020

Explain this line.
23. ### Challenge Math Challenge - August 2020

You really need that ##g(\Bbb{C}^*) = \Bbb{C}^*## for your argument to work. Also, when using the symbol ##\cong##, explain what you mean with the symbol. I assume that you mean isomorphism in the category of continuous maps, but how are you even sure the inverse on the image is continuous as well?
24. ### Challenge Math Challenge - August 2020

Hmm, I need a little more details here. Explain why the discontinuities arise. I find your argument a bit too handwavy.
25. ### Challenge Math Challenge - August 2020

Seems to work! Working with diagonal operators was also what I had in mind. Here is a more sophisticated approach using the theory of ##C^*##-algebras: Consider the ##C^*##-algebra ##C(K)##. Then the inclusion ##i: K \to \Bbb{C}## has spectrum ##K##. Next, choose a Hilbert space ##H## and an...
26. ### Challenge Math Challenge - August 2020

Yes, the idea is indeed that on the x-axis trouble arise, but I don't think what you wrote is rigorous enough.
27. ### Challenge Math Challenge - August 2020

Yes, the problem I see with this is that for different choices of ##z## you can have different values of ##n##. So you should write ##n= n(z)##.
28. ### Challenge Math Challenge - August 2020

But the plane ##\Bbb{C}## with the origin removed is not simply connected (consider fundamental group), so the theorem does not apply. Also, if the plane was simply connected your theorem would contradict the exercise.
29. ### Challenge Math Challenge - August 2020

There are multiple approaches. First, you need to say what convention you use: I guess you write ##z=r e^ {i \theta}##. What domain do you allow for ##\theta##? Then, why does ##g## take that form and why does it have a discontinuity?
30. ### Challenge Math Challenge - August 2020

Looks correct! Well done!