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Undefined at ##0## doesn't mean discontinuous at ##0##Isaac0427 said:##y=\frac{x}{x}## is differentiable, and
it's derivative, using the quotient rule, is ##\frac{0}{x^2}##, which is undefined at x=0 and thus not continuous.
Undefined at ##0## doesn't mean discontinuous at ##0##Isaac0427 said:##y=\frac{x}{x}## is differentiable, and
it's derivative, using the quotient rule, is ##\frac{0}{x^2}##, which is undefined at x=0 and thus not continuous.
stevendaryl said:I think I know how to solve #3.Suppose we let [itex]f^2(x) = \sum_{n=0}^{\infty} e^{-\lambda_n (x-n)^2}[/itex], where [itex]\lambda_n[/itex] is a sequence of constants. (So [itex]f(x)[/itex] is the square-root of the sum). Then it would certainly not be the case that [itex]lim_{x \rightarrow +\infty} f(x) = 0[/itex]. But the integral might still be finite:
[itex]\int dx f^2(x) = \sum_{n=0}^{\infty} \int dx e^{-\lambda_n (x-n)^2} = \sum_{n=0}^{\infty} \sqrt{\frac{\pi}{\lambda_n}}[/itex]
This sum converges, provided that [itex]\lambda_n[/itex] grows sufficiently rapidly. For example, [itex]\lambda_n = 2^n[/itex].
So my answer is: [itex]f(x) = \sqrt{ \sum_{n=0}^{\infty} e^{-2^n (x-n)^2}}[/itex]
You'd have to prove that [itex]f(x)[/itex] is well-defined and differentiable, but I think it's true.
limx→0f(x)≠f(0) If f(x)=##\frac{0}{x^2}##.micromass said:Undefined at ##0## doesn't mean discontinuous at ##0##
Isaac0427 said:limx→0f(x)≠f(0) If f(x)=##\frac{0}{x^2}##.
I know that @ProfuselyQuarky solved it before me, just want to know why I'm wrong.
Ah ok.micromass said:Because ##0## is not in the domain of your function. It's true that ##f## is continuous at ##0## iff ##\lim_{x\rightarrow 0}f(x) = f(0)##, but that is only when ##0## is actually in the domain of your function.
micromass said:Cool idea, but I can't really accept this solution until some gaps are filled:
- Prove ##f(x)## is well-defined
- Prove ##f(x)## is differentiable.
- Show why ##\lim_{x\rightarrow +\infty} f(x) =0##
- Show that the interchange of series and integral is justified.
stevendaryl said:Oh, bother! The idea is to have a function that is basically a bunch of "spikes" that always have height 1, but whose widths decrease. Gaussians have that property, but working with them is a little bit of a pain. But here's another choice:
stevendaryl said:Oh, bother! The idea is to have a function that is basically a bunch of "spikes" that always have height 1, but whose widths decrease. Gaussians have that property, but working with them is a little bit of a pain. But here's another choice:
First define [itex]g(x)[/itex] as follows:
[itex]g(x) = 0[/itex] if [itex]x < -\frac{1}{2}[/itex]
[itex]g(x) = 1 + cos(2\pi x)[/itex] if [itex]-\frac{1}{2} < x < +\frac{1}{2}[/itex]
[itex]g(x) = 0[/itex] if [itex]x > \frac{1}{2}[/itex]
Then [itex]g(x)[/itex] is continuous and once-differentiable.
Then [itex]\int_{-\infty}^{+\infty} g(\lambda x) dx = \frac{1}{\lambda} \int_{-\frac{1}{2}}^{+\frac{1}{2}} (1+cos(2\pi x)) dx = \frac{1}{\lambda}[/itex]
Let [itex]\lambda_n[/itex] be any sequence of real numbers greater than 1 such that [itex]\sum_n \frac{1}{\lambda_n} < \infty[/itex]
Define [itex]g_n(x) = g(\lambda_n (x-n))[/itex]. Then for any [itex]n \neq m[/itex], either [itex]g_n(x) = 0[/itex] or [itex]g_m(x) = 0[/itex] or both. Furthermore, [itex]g_n(n+\frac{1}{2}) = g_{n+1}(n+\frac{1}{2}) = 0[/itex].
Let [itex]N[x][/itex] = the smallest [itex]n[/itex] such that [itex]n+\frac{1}{2} > x[/itex]
So now define [itex]q(x) = g_{N[x]}(x)[/itex], where [itex]n[/itex] is the smallest number such that [itex]n+\frac{1}{2} > x[/itex]. Then [itex]q(x)[/itex] is continuous: For [itex]x[/itex] in the range [itex]n-\frac{1}{2} < x < n+\frac{1}{2}[/itex], [itex]q(x) = g_n(x)[/itex], which is continuous. At [itex]x=n+\frac{1}{2}[/itex], we have:
[itex]q(x-\epsilon) = g_n(x-\epsilon) = 0[/itex]
[itex]q(x+\epsilon) = g_{n+1}(x+\epsilon) = 0[/itex]
[itex]q(x)[/itex] is also once-differentiable, since [itex]g_n(x)[/itex] is, and the derivatives are zero at the boundary.
Now, [itex]\int_{-\infty}^{+\infty} q(x) dx = \sum_n \int_{n-\frac{1}{2}}^{n+\frac{1}{2}} q(x) dx = \sum_n \int_{n-\frac{1}{2}}^{n+\frac{1}{2}} g_n(x) dx = \sum_n \frac{1}{\lambda_n} < \infty[/itex]
andrewkirk said:Why do I think my answer to 10 works? Well the argument as to why a topologists' sine curve is connected is known.
Well that's the point of the counterexample (and the point of the topologists' sine curve): to exemplify the difference between path connectedness and mere connectedness. I think it would be straightforward to prove that the proposition is true (ie there is no counterexample) if one replaces 'connected' by 'path-connected'.stevendaryl said:Yeah, but to me it means that the definition of "connected" isn't quite right. An alternative definition is "path connected", which is that you can from any point to any other point through a continuous one-dimensional path.
The function ##f:\mathbb{R}\to\mathbb R## such that, for ##x>0##:micromass said:4.There is no infinitely differentiable function ##f:\mathbb{R}\rightarrow \mathbb{R}## such that ##f(x) = 0## if and only if ##x\in \{1/n~\vert~n\in \mathbb{N}\}\cup\{0\}##
Define ##f:\mathbb{R}^2\to\mathbb{R}## bymicromass said:9. If ##f:\mathbb{R}^2\rightarrow \mathbb{R}## is a function such that ##\lim_{(x,y)\rightarrow (0,0)} f(x,y)## exists and is finite, then both ##\lim_{x\rightarrow 0}\lim_{y\rightarrow 0} f(x,y)## and ##\lim_{y\rightarrow 0}\lim_{x\rightarrow 0} f(x,y)## exist and are finite.
A proof occurred to me. At first I thought we could use the intermediate value theorem on the two paths, until I realized that they don't necessarily map out the locus of functions from ##\mathbb R## to ##\mathbb R##. We can adapt the curves to remove 'turn-backs' so they become functions, but we lose continuity, which is necessary to use the IVT.andrewkirk said:I think it would be straightforward to prove that the proposition is true (ie there is no counterexample) if one replaces 'connected' by 'path-connected'.
There's a second part: https://www.physicsforums.com/threads/yet-another-counterexample-challenge.869304/mfb said:I missed this thread :(.
Can you add links to the posted solutions to the first post?
Misconception alert!micromass said:SOLVED BY fresh_42 If ##(a_n)_n## is a sequence such that for each positive integer ##p## holds that ##\lim_{n\rightarrow +\infty} a_{n+p} - a_n = 0##, then ##a_n## converges.
It is not. The order of ##\epsilon##, N and p is different, and this difference matters.Abdullah Naeem said:Is this the same as "for all ##\epsilon >0##, there exists ##N,p## such that ##d(x_{n+p},x_n)< \epsilon## whenever ##n \geq N## "?
Doesn't this work? : Consider any irrational x. As singletons ( and finite sets ) are open, then consider its complement in the Real line: ## \mathbb R ##- { ##x ##}, which is open as the complement of the closed set { ## x ##}? And, since we only removed an irrational, ## \mathbb Q ## is contained in it. This gives us uncountably-many (counter) examples.Samy_A said:Nice, some I know, some not.
Let me try 1:
Any open set ##G## such that ##\mathbb{Q}\subseteq G\subseteq \mathbb{R}## has either ##G=\mathbb{Q}## or ##G=\mathbb{R}##.
Set ##\mathbb{Q} = \{ q_1,q_2,q_3, ...\}##
Let's build an open set ##G## such that ##\mathbb{Q}\subseteq G## that can be written as ##G=\cup ]q_n-\epsilon_n,q_n+\epsilon_n[##.
##\mathbb{Q}\subseteq G## is trivially true. That ##G## is open is also trivial, as ##G## is the union of open sets.
Also, any open interval in ##\mathbb R## will contain irrational numbers, so that ##\mathbb{Q} \neq G##.
Now, we don't want ##G=\mathbb{R}##.
So let's build the ##\epsilon_n## such that some chosen non rational number is not in ##G##.
##\pi## is not a rational number.
Set ##\displaystyle \epsilon_n=\frac{1}{2}\min_{m \leq n} |\pi -q_m|##.
Clearly ##\epsilon_n>0##. Also ##\pi \notin ]q_n-\epsilon_n,q_n+\epsilon_n[##, since ##|\pi- q_n| \geq 2\epsilon_n \gt \epsilon_n## (and this for all ##n##).
Hence ##\pi \notin G##.
EDIT: I think my definition of the ##\epsilon_n## is too complicated.
##\displaystyle \epsilon_n=\frac{1}{2}|\pi -q_n|## would also work.
That is basically what @Samy_A constructed, just with a much shorter argument.WWGD said:Doesn't this work? : Consider any irrational x. As singletons ( and finite sets ) are open, then consider its complement in the Real line: ## \mathbb R ##- { ##x ##}, which is open as the complement of the closed set { ## x ##}? And, since we only removed an irrational, ## \mathbb Q ## is contained in it. This gives us uncountably-many (counter) examples.
Didn't he remove whole intervals? I am saying:mfb said:That is basically what @Samy_A constructed, just with a much shorter argument.
If you take the union of all his sets (to get the final set) you should get the same with both approaches.WWGD said:Didn't he remove whole intervals?
o yes, especially for those who did not read Bernard R. Gelbaum, John M. H. Olmsted: Counterexamples in Analysis. :)micromass said:I hope some of these statements were surprising to some of you