Polynomial growth implies locally Lipschitz?
Clash Royale CLAN TAG#URR8PPP
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1
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Let $f:mathbbR^mtomathbbR^m$ satisfy $|f(x)|le c|x|^n$ for some (re-edit:) $ninmathbbN$ with some constant $c>0$. Is $f$ locally Lipschitz?
I see that it is around $x=0$: $|f(x)-f(0)|le frac^n+c0|x|=c|x|^n-1|x|le delta |x|$ for a suitable $delta$.
Apparently, a polarization argument is a way to go but I do not know how to apply it for $|f(x)-f(x')|$.
real-analysis lipschitz-functions nonlinear-analysis
add a comment |Â
up vote
1
down vote
favorite
Let $f:mathbbR^mtomathbbR^m$ satisfy $|f(x)|le c|x|^n$ for some (re-edit:) $ninmathbbN$ with some constant $c>0$. Is $f$ locally Lipschitz?
I see that it is around $x=0$: $|f(x)-f(0)|le frac^n+c0|x|=c|x|^n-1|x|le delta |x|$ for a suitable $delta$.
Apparently, a polarization argument is a way to go but I do not know how to apply it for $|f(x)-f(x')|$.
real-analysis lipschitz-functions nonlinear-analysis
What definition of "locally Lipschitz" are you using?
â zhw.
27 mins ago
add a comment |Â
up vote
1
down vote
favorite
up vote
1
down vote
favorite
Let $f:mathbbR^mtomathbbR^m$ satisfy $|f(x)|le c|x|^n$ for some (re-edit:) $ninmathbbN$ with some constant $c>0$. Is $f$ locally Lipschitz?
I see that it is around $x=0$: $|f(x)-f(0)|le frac^n+c0|x|=c|x|^n-1|x|le delta |x|$ for a suitable $delta$.
Apparently, a polarization argument is a way to go but I do not know how to apply it for $|f(x)-f(x')|$.
real-analysis lipschitz-functions nonlinear-analysis
Let $f:mathbbR^mtomathbbR^m$ satisfy $|f(x)|le c|x|^n$ for some (re-edit:) $ninmathbbN$ with some constant $c>0$. Is $f$ locally Lipschitz?
I see that it is around $x=0$: $|f(x)-f(0)|le frac^n+c0|x|=c|x|^n-1|x|le delta |x|$ for a suitable $delta$.
Apparently, a polarization argument is a way to go but I do not know how to apply it for $|f(x)-f(x')|$.
real-analysis lipschitz-functions nonlinear-analysis
real-analysis lipschitz-functions nonlinear-analysis
edited 1 hour ago
asked 1 hour ago
shall.i.am
366217
366217
What definition of "locally Lipschitz" are you using?
â zhw.
27 mins ago
add a comment |Â
What definition of "locally Lipschitz" are you using?
â zhw.
27 mins ago
What definition of "locally Lipschitz" are you using?
â zhw.
27 mins ago
What definition of "locally Lipschitz" are you using?
â zhw.
27 mins ago
add a comment |Â
3 Answers
3
active
oldest
votes
up vote
2
down vote
I think the function $f$ from $mathbbR$ to $mathbbR$ define by $f(x)=x sin (1/x)$ is a counter example to your problem.
New contributor
2
This is not a counterexample, because local Lipschitz allows constants be dependent on $x$.
â Hans
1 hour ago
2
There is nothing to reconcile. The definition of local Lipschitz en.wikipedia.org/wiki/Lipschitz_continuity#Definitions is satisfied for this example in the answer.
â Hans
1 hour ago
Ah, that's what I thought. Great, thanks. The example "Differentiable functions that are not (locally) Lipschitz continuous" in the wikipedia page is very confusing.
â shall.i.am
1 hour ago
1
@Hans The definition of "locally Lipschitz" you cite requires that for each $x,$ there exists a full neighborhood $U_x$ of $x$ in which $f$ is Lipschitz. There is no such neighborhood of $0$ for the example of AlexL, so it is a valid counterexample.
â zhw.
36 mins ago
1
@Surb No, the example offered by AlexL is not locally Lipschitz. Perhaps you are thinking of "pointwise Lipschitz"?
â zhw.
34 mins ago
 |Â
show 5 more comments
up vote
1
down vote
A counterexample is
$$f(x)=begincases |x|^n-1x, |x|inmathbb Q\
-|x|^n-1x, |x|notinmathbb Q
endcases.
$$
Is it obvious that this is a counter example ?
â Surb
39 mins ago
1
@Surb: Yes, since it is discontinuous.
â Hans
38 mins ago
1
@Surb: I have modified my counterexample for it to work for arbitrary $m$.
â Hans
26 mins ago
I have already upvoted your answer :)
â Surb
26 mins ago
@Surb: I understand. Thank you. :-) I just want to address your previous concern for $m>1$.
â Hans
21 mins ago
add a comment |Â
up vote
1
down vote
A continuous nowhere differentiable function such as
$$sum_n=1^infty sin(2^n x)over n^2$$
is bounded but it most certainly is not locally lipschitz.
really? continuous implies uniform continuity on any compact isn't it?
â Surb
26 mins ago
This is uniformly continuous, but it is not locally lipschitz. That is a far stronger condition than continuity.
â ncmathsadist
24 mins ago
right makes sense, I mixed up two definitions$
â Surb
23 mins ago
I like the example but can you prove it is not locally Lipschitz or provide a reference to the proof of this claim?
â Hans
11 mins ago
add a comment |Â
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
I think the function $f$ from $mathbbR$ to $mathbbR$ define by $f(x)=x sin (1/x)$ is a counter example to your problem.
New contributor
2
This is not a counterexample, because local Lipschitz allows constants be dependent on $x$.
â Hans
1 hour ago
2
There is nothing to reconcile. The definition of local Lipschitz en.wikipedia.org/wiki/Lipschitz_continuity#Definitions is satisfied for this example in the answer.
â Hans
1 hour ago
Ah, that's what I thought. Great, thanks. The example "Differentiable functions that are not (locally) Lipschitz continuous" in the wikipedia page is very confusing.
â shall.i.am
1 hour ago
1
@Hans The definition of "locally Lipschitz" you cite requires that for each $x,$ there exists a full neighborhood $U_x$ of $x$ in which $f$ is Lipschitz. There is no such neighborhood of $0$ for the example of AlexL, so it is a valid counterexample.
â zhw.
36 mins ago
1
@Surb No, the example offered by AlexL is not locally Lipschitz. Perhaps you are thinking of "pointwise Lipschitz"?
â zhw.
34 mins ago
 |Â
show 5 more comments
up vote
2
down vote
I think the function $f$ from $mathbbR$ to $mathbbR$ define by $f(x)=x sin (1/x)$ is a counter example to your problem.
New contributor
2
This is not a counterexample, because local Lipschitz allows constants be dependent on $x$.
â Hans
1 hour ago
2
There is nothing to reconcile. The definition of local Lipschitz en.wikipedia.org/wiki/Lipschitz_continuity#Definitions is satisfied for this example in the answer.
â Hans
1 hour ago
Ah, that's what I thought. Great, thanks. The example "Differentiable functions that are not (locally) Lipschitz continuous" in the wikipedia page is very confusing.
â shall.i.am
1 hour ago
1
@Hans The definition of "locally Lipschitz" you cite requires that for each $x,$ there exists a full neighborhood $U_x$ of $x$ in which $f$ is Lipschitz. There is no such neighborhood of $0$ for the example of AlexL, so it is a valid counterexample.
â zhw.
36 mins ago
1
@Surb No, the example offered by AlexL is not locally Lipschitz. Perhaps you are thinking of "pointwise Lipschitz"?
â zhw.
34 mins ago
 |Â
show 5 more comments
up vote
2
down vote
up vote
2
down vote
I think the function $f$ from $mathbbR$ to $mathbbR$ define by $f(x)=x sin (1/x)$ is a counter example to your problem.
New contributor
I think the function $f$ from $mathbbR$ to $mathbbR$ define by $f(x)=x sin (1/x)$ is a counter example to your problem.
New contributor
New contributor
answered 1 hour ago
AlexL
591
591
New contributor
New contributor
2
This is not a counterexample, because local Lipschitz allows constants be dependent on $x$.
â Hans
1 hour ago
2
There is nothing to reconcile. The definition of local Lipschitz en.wikipedia.org/wiki/Lipschitz_continuity#Definitions is satisfied for this example in the answer.
â Hans
1 hour ago
Ah, that's what I thought. Great, thanks. The example "Differentiable functions that are not (locally) Lipschitz continuous" in the wikipedia page is very confusing.
â shall.i.am
1 hour ago
1
@Hans The definition of "locally Lipschitz" you cite requires that for each $x,$ there exists a full neighborhood $U_x$ of $x$ in which $f$ is Lipschitz. There is no such neighborhood of $0$ for the example of AlexL, so it is a valid counterexample.
â zhw.
36 mins ago
1
@Surb No, the example offered by AlexL is not locally Lipschitz. Perhaps you are thinking of "pointwise Lipschitz"?
â zhw.
34 mins ago
 |Â
show 5 more comments
2
This is not a counterexample, because local Lipschitz allows constants be dependent on $x$.
â Hans
1 hour ago
2
There is nothing to reconcile. The definition of local Lipschitz en.wikipedia.org/wiki/Lipschitz_continuity#Definitions is satisfied for this example in the answer.
â Hans
1 hour ago
Ah, that's what I thought. Great, thanks. The example "Differentiable functions that are not (locally) Lipschitz continuous" in the wikipedia page is very confusing.
â shall.i.am
1 hour ago
1
@Hans The definition of "locally Lipschitz" you cite requires that for each $x,$ there exists a full neighborhood $U_x$ of $x$ in which $f$ is Lipschitz. There is no such neighborhood of $0$ for the example of AlexL, so it is a valid counterexample.
â zhw.
36 mins ago
1
@Surb No, the example offered by AlexL is not locally Lipschitz. Perhaps you are thinking of "pointwise Lipschitz"?
â zhw.
34 mins ago
2
2
This is not a counterexample, because local Lipschitz allows constants be dependent on $x$.
â Hans
1 hour ago
This is not a counterexample, because local Lipschitz allows constants be dependent on $x$.
â Hans
1 hour ago
2
2
There is nothing to reconcile. The definition of local Lipschitz en.wikipedia.org/wiki/Lipschitz_continuity#Definitions is satisfied for this example in the answer.
â Hans
1 hour ago
There is nothing to reconcile. The definition of local Lipschitz en.wikipedia.org/wiki/Lipschitz_continuity#Definitions is satisfied for this example in the answer.
â Hans
1 hour ago
Ah, that's what I thought. Great, thanks. The example "Differentiable functions that are not (locally) Lipschitz continuous" in the wikipedia page is very confusing.
â shall.i.am
1 hour ago
Ah, that's what I thought. Great, thanks. The example "Differentiable functions that are not (locally) Lipschitz continuous" in the wikipedia page is very confusing.
â shall.i.am
1 hour ago
1
1
@Hans The definition of "locally Lipschitz" you cite requires that for each $x,$ there exists a full neighborhood $U_x$ of $x$ in which $f$ is Lipschitz. There is no such neighborhood of $0$ for the example of AlexL, so it is a valid counterexample.
â zhw.
36 mins ago
@Hans The definition of "locally Lipschitz" you cite requires that for each $x,$ there exists a full neighborhood $U_x$ of $x$ in which $f$ is Lipschitz. There is no such neighborhood of $0$ for the example of AlexL, so it is a valid counterexample.
â zhw.
36 mins ago
1
1
@Surb No, the example offered by AlexL is not locally Lipschitz. Perhaps you are thinking of "pointwise Lipschitz"?
â zhw.
34 mins ago
@Surb No, the example offered by AlexL is not locally Lipschitz. Perhaps you are thinking of "pointwise Lipschitz"?
â zhw.
34 mins ago
 |Â
show 5 more comments
up vote
1
down vote
A counterexample is
$$f(x)=begincases |x|^n-1x, |x|inmathbb Q\
-|x|^n-1x, |x|notinmathbb Q
endcases.
$$
Is it obvious that this is a counter example ?
â Surb
39 mins ago
1
@Surb: Yes, since it is discontinuous.
â Hans
38 mins ago
1
@Surb: I have modified my counterexample for it to work for arbitrary $m$.
â Hans
26 mins ago
I have already upvoted your answer :)
â Surb
26 mins ago
@Surb: I understand. Thank you. :-) I just want to address your previous concern for $m>1$.
â Hans
21 mins ago
add a comment |Â
up vote
1
down vote
A counterexample is
$$f(x)=begincases |x|^n-1x, |x|inmathbb Q\
-|x|^n-1x, |x|notinmathbb Q
endcases.
$$
Is it obvious that this is a counter example ?
â Surb
39 mins ago
1
@Surb: Yes, since it is discontinuous.
â Hans
38 mins ago
1
@Surb: I have modified my counterexample for it to work for arbitrary $m$.
â Hans
26 mins ago
I have already upvoted your answer :)
â Surb
26 mins ago
@Surb: I understand. Thank you. :-) I just want to address your previous concern for $m>1$.
â Hans
21 mins ago
add a comment |Â
up vote
1
down vote
up vote
1
down vote
A counterexample is
$$f(x)=begincases |x|^n-1x, |x|inmathbb Q\
-|x|^n-1x, |x|notinmathbb Q
endcases.
$$
A counterexample is
$$f(x)=begincases |x|^n-1x, |x|inmathbb Q\
-|x|^n-1x, |x|notinmathbb Q
endcases.
$$
edited 28 mins ago
answered 50 mins ago
Hans
4,42621029
4,42621029
Is it obvious that this is a counter example ?
â Surb
39 mins ago
1
@Surb: Yes, since it is discontinuous.
â Hans
38 mins ago
1
@Surb: I have modified my counterexample for it to work for arbitrary $m$.
â Hans
26 mins ago
I have already upvoted your answer :)
â Surb
26 mins ago
@Surb: I understand. Thank you. :-) I just want to address your previous concern for $m>1$.
â Hans
21 mins ago
add a comment |Â
Is it obvious that this is a counter example ?
â Surb
39 mins ago
1
@Surb: Yes, since it is discontinuous.
â Hans
38 mins ago
1
@Surb: I have modified my counterexample for it to work for arbitrary $m$.
â Hans
26 mins ago
I have already upvoted your answer :)
â Surb
26 mins ago
@Surb: I understand. Thank you. :-) I just want to address your previous concern for $m>1$.
â Hans
21 mins ago
Is it obvious that this is a counter example ?
â Surb
39 mins ago
Is it obvious that this is a counter example ?
â Surb
39 mins ago
1
1
@Surb: Yes, since it is discontinuous.
â Hans
38 mins ago
@Surb: Yes, since it is discontinuous.
â Hans
38 mins ago
1
1
@Surb: I have modified my counterexample for it to work for arbitrary $m$.
â Hans
26 mins ago
@Surb: I have modified my counterexample for it to work for arbitrary $m$.
â Hans
26 mins ago
I have already upvoted your answer :)
â Surb
26 mins ago
I have already upvoted your answer :)
â Surb
26 mins ago
@Surb: I understand. Thank you. :-) I just want to address your previous concern for $m>1$.
â Hans
21 mins ago
@Surb: I understand. Thank you. :-) I just want to address your previous concern for $m>1$.
â Hans
21 mins ago
add a comment |Â
up vote
1
down vote
A continuous nowhere differentiable function such as
$$sum_n=1^infty sin(2^n x)over n^2$$
is bounded but it most certainly is not locally lipschitz.
really? continuous implies uniform continuity on any compact isn't it?
â Surb
26 mins ago
This is uniformly continuous, but it is not locally lipschitz. That is a far stronger condition than continuity.
â ncmathsadist
24 mins ago
right makes sense, I mixed up two definitions$
â Surb
23 mins ago
I like the example but can you prove it is not locally Lipschitz or provide a reference to the proof of this claim?
â Hans
11 mins ago
add a comment |Â
up vote
1
down vote
A continuous nowhere differentiable function such as
$$sum_n=1^infty sin(2^n x)over n^2$$
is bounded but it most certainly is not locally lipschitz.
really? continuous implies uniform continuity on any compact isn't it?
â Surb
26 mins ago
This is uniformly continuous, but it is not locally lipschitz. That is a far stronger condition than continuity.
â ncmathsadist
24 mins ago
right makes sense, I mixed up two definitions$
â Surb
23 mins ago
I like the example but can you prove it is not locally Lipschitz or provide a reference to the proof of this claim?
â Hans
11 mins ago
add a comment |Â
up vote
1
down vote
up vote
1
down vote
A continuous nowhere differentiable function such as
$$sum_n=1^infty sin(2^n x)over n^2$$
is bounded but it most certainly is not locally lipschitz.
A continuous nowhere differentiable function such as
$$sum_n=1^infty sin(2^n x)over n^2$$
is bounded but it most certainly is not locally lipschitz.
answered 27 mins ago
ncmathsadist
41.5k258100
41.5k258100
really? continuous implies uniform continuity on any compact isn't it?
â Surb
26 mins ago
This is uniformly continuous, but it is not locally lipschitz. That is a far stronger condition than continuity.
â ncmathsadist
24 mins ago
right makes sense, I mixed up two definitions$
â Surb
23 mins ago
I like the example but can you prove it is not locally Lipschitz or provide a reference to the proof of this claim?
â Hans
11 mins ago
add a comment |Â
really? continuous implies uniform continuity on any compact isn't it?
â Surb
26 mins ago
This is uniformly continuous, but it is not locally lipschitz. That is a far stronger condition than continuity.
â ncmathsadist
24 mins ago
right makes sense, I mixed up two definitions$
â Surb
23 mins ago
I like the example but can you prove it is not locally Lipschitz or provide a reference to the proof of this claim?
â Hans
11 mins ago
really? continuous implies uniform continuity on any compact isn't it?
â Surb
26 mins ago
really? continuous implies uniform continuity on any compact isn't it?
â Surb
26 mins ago
This is uniformly continuous, but it is not locally lipschitz. That is a far stronger condition than continuity.
â ncmathsadist
24 mins ago
This is uniformly continuous, but it is not locally lipschitz. That is a far stronger condition than continuity.
â ncmathsadist
24 mins ago
right makes sense, I mixed up two definitions$
â Surb
23 mins ago
right makes sense, I mixed up two definitions$
â Surb
23 mins ago
I like the example but can you prove it is not locally Lipschitz or provide a reference to the proof of this claim?
â Hans
11 mins ago
I like the example but can you prove it is not locally Lipschitz or provide a reference to the proof of this claim?
â Hans
11 mins ago
add a comment |Â
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What definition of "locally Lipschitz" are you using?
â zhw.
27 mins ago