Convergence in L^2












2












$begingroup$



Define a function $f_varepsilon: mathbb{R} to mathbb{R}$ as
$$begin{equation}
f_varepsilon(x) =
begin{cases}
-1 & text{if}; ,x<-varepsilon \
sin left(frac{pi x}{2 varepsilon}right) & text{if};, |x| le varepsilon\
1 & text{if};, x>varepsilon
end{cases}
end{equation} $$

Then $f_varepsilon(x)$ converges pointwise to $mbox{sgn}(x)$ as $varepsilon to 0$. And it can be easily proven that $ int_{mathbb{R}}|f_varepsilon(x)- mbox{sgn}(x)|^2,dx= Cvarepsilon $ .




My question is given $u in L^1(mathbb{R}) cap L^2(mathbb{R})$ and $varphi in C^infty_c(mathbb{R})$ can we prove $$int_{mathbb{R}}|f_varepsilon(u(x))- mbox{sgn}(u(x))|^2varphi(x),dx le Cvarepsilon^r$$ for some r>0.
I actually tried to mimic the proof but the main issue over here is we cannot apply the change of variable formula. And I'm always ending up in an upper bound like $varepsilon^r+c$, which is not desired.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    What would $delta$ be?
    $endgroup$
    – Mindlack
    Jan 9 at 11:40










  • $begingroup$
    If the fact you can't apply change of variable formula brothers you, I'd suggest try to prove it first for u smooth and then just approach your general function by smooth functions on the support of $phi$.
    $endgroup$
    – Keen
    Jan 9 at 13:42












  • $begingroup$
    @Mindlack I'm sorry that would be epsilon.
    $endgroup$
    – Goal123
    Jan 9 at 16:37










  • $begingroup$
    What then if $u=0$?
    $endgroup$
    – Mindlack
    Jan 9 at 16:54






  • 1




    $begingroup$
    @Keen $u$ is required to be one-one to apply change of variable, which need not be true here.
    $endgroup$
    – Goal123
    Jan 9 at 17:59
















2












$begingroup$



Define a function $f_varepsilon: mathbb{R} to mathbb{R}$ as
$$begin{equation}
f_varepsilon(x) =
begin{cases}
-1 & text{if}; ,x<-varepsilon \
sin left(frac{pi x}{2 varepsilon}right) & text{if};, |x| le varepsilon\
1 & text{if};, x>varepsilon
end{cases}
end{equation} $$

Then $f_varepsilon(x)$ converges pointwise to $mbox{sgn}(x)$ as $varepsilon to 0$. And it can be easily proven that $ int_{mathbb{R}}|f_varepsilon(x)- mbox{sgn}(x)|^2,dx= Cvarepsilon $ .




My question is given $u in L^1(mathbb{R}) cap L^2(mathbb{R})$ and $varphi in C^infty_c(mathbb{R})$ can we prove $$int_{mathbb{R}}|f_varepsilon(u(x))- mbox{sgn}(u(x))|^2varphi(x),dx le Cvarepsilon^r$$ for some r>0.
I actually tried to mimic the proof but the main issue over here is we cannot apply the change of variable formula. And I'm always ending up in an upper bound like $varepsilon^r+c$, which is not desired.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    What would $delta$ be?
    $endgroup$
    – Mindlack
    Jan 9 at 11:40










  • $begingroup$
    If the fact you can't apply change of variable formula brothers you, I'd suggest try to prove it first for u smooth and then just approach your general function by smooth functions on the support of $phi$.
    $endgroup$
    – Keen
    Jan 9 at 13:42












  • $begingroup$
    @Mindlack I'm sorry that would be epsilon.
    $endgroup$
    – Goal123
    Jan 9 at 16:37










  • $begingroup$
    What then if $u=0$?
    $endgroup$
    – Mindlack
    Jan 9 at 16:54






  • 1




    $begingroup$
    @Keen $u$ is required to be one-one to apply change of variable, which need not be true here.
    $endgroup$
    – Goal123
    Jan 9 at 17:59














2












2








2





$begingroup$



Define a function $f_varepsilon: mathbb{R} to mathbb{R}$ as
$$begin{equation}
f_varepsilon(x) =
begin{cases}
-1 & text{if}; ,x<-varepsilon \
sin left(frac{pi x}{2 varepsilon}right) & text{if};, |x| le varepsilon\
1 & text{if};, x>varepsilon
end{cases}
end{equation} $$

Then $f_varepsilon(x)$ converges pointwise to $mbox{sgn}(x)$ as $varepsilon to 0$. And it can be easily proven that $ int_{mathbb{R}}|f_varepsilon(x)- mbox{sgn}(x)|^2,dx= Cvarepsilon $ .




My question is given $u in L^1(mathbb{R}) cap L^2(mathbb{R})$ and $varphi in C^infty_c(mathbb{R})$ can we prove $$int_{mathbb{R}}|f_varepsilon(u(x))- mbox{sgn}(u(x))|^2varphi(x),dx le Cvarepsilon^r$$ for some r>0.
I actually tried to mimic the proof but the main issue over here is we cannot apply the change of variable formula. And I'm always ending up in an upper bound like $varepsilon^r+c$, which is not desired.










share|cite|improve this question











$endgroup$





Define a function $f_varepsilon: mathbb{R} to mathbb{R}$ as
$$begin{equation}
f_varepsilon(x) =
begin{cases}
-1 & text{if}; ,x<-varepsilon \
sin left(frac{pi x}{2 varepsilon}right) & text{if};, |x| le varepsilon\
1 & text{if};, x>varepsilon
end{cases}
end{equation} $$

Then $f_varepsilon(x)$ converges pointwise to $mbox{sgn}(x)$ as $varepsilon to 0$. And it can be easily proven that $ int_{mathbb{R}}|f_varepsilon(x)- mbox{sgn}(x)|^2,dx= Cvarepsilon $ .




My question is given $u in L^1(mathbb{R}) cap L^2(mathbb{R})$ and $varphi in C^infty_c(mathbb{R})$ can we prove $$int_{mathbb{R}}|f_varepsilon(u(x))- mbox{sgn}(u(x))|^2varphi(x),dx le Cvarepsilon^r$$ for some r>0.
I actually tried to mimic the proof but the main issue over here is we cannot apply the change of variable formula. And I'm always ending up in an upper bound like $varepsilon^r+c$, which is not desired.







analysis






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 9 at 16:37







Goal123

















asked Jan 9 at 11:33









Goal123Goal123

507212




507212








  • 1




    $begingroup$
    What would $delta$ be?
    $endgroup$
    – Mindlack
    Jan 9 at 11:40










  • $begingroup$
    If the fact you can't apply change of variable formula brothers you, I'd suggest try to prove it first for u smooth and then just approach your general function by smooth functions on the support of $phi$.
    $endgroup$
    – Keen
    Jan 9 at 13:42












  • $begingroup$
    @Mindlack I'm sorry that would be epsilon.
    $endgroup$
    – Goal123
    Jan 9 at 16:37










  • $begingroup$
    What then if $u=0$?
    $endgroup$
    – Mindlack
    Jan 9 at 16:54






  • 1




    $begingroup$
    @Keen $u$ is required to be one-one to apply change of variable, which need not be true here.
    $endgroup$
    – Goal123
    Jan 9 at 17:59














  • 1




    $begingroup$
    What would $delta$ be?
    $endgroup$
    – Mindlack
    Jan 9 at 11:40










  • $begingroup$
    If the fact you can't apply change of variable formula brothers you, I'd suggest try to prove it first for u smooth and then just approach your general function by smooth functions on the support of $phi$.
    $endgroup$
    – Keen
    Jan 9 at 13:42












  • $begingroup$
    @Mindlack I'm sorry that would be epsilon.
    $endgroup$
    – Goal123
    Jan 9 at 16:37










  • $begingroup$
    What then if $u=0$?
    $endgroup$
    – Mindlack
    Jan 9 at 16:54






  • 1




    $begingroup$
    @Keen $u$ is required to be one-one to apply change of variable, which need not be true here.
    $endgroup$
    – Goal123
    Jan 9 at 17:59








1




1




$begingroup$
What would $delta$ be?
$endgroup$
– Mindlack
Jan 9 at 11:40




$begingroup$
What would $delta$ be?
$endgroup$
– Mindlack
Jan 9 at 11:40












$begingroup$
If the fact you can't apply change of variable formula brothers you, I'd suggest try to prove it first for u smooth and then just approach your general function by smooth functions on the support of $phi$.
$endgroup$
– Keen
Jan 9 at 13:42






$begingroup$
If the fact you can't apply change of variable formula brothers you, I'd suggest try to prove it first for u smooth and then just approach your general function by smooth functions on the support of $phi$.
$endgroup$
– Keen
Jan 9 at 13:42














$begingroup$
@Mindlack I'm sorry that would be epsilon.
$endgroup$
– Goal123
Jan 9 at 16:37




$begingroup$
@Mindlack I'm sorry that would be epsilon.
$endgroup$
– Goal123
Jan 9 at 16:37












$begingroup$
What then if $u=0$?
$endgroup$
– Mindlack
Jan 9 at 16:54




$begingroup$
What then if $u=0$?
$endgroup$
– Mindlack
Jan 9 at 16:54




1




1




$begingroup$
@Keen $u$ is required to be one-one to apply change of variable, which need not be true here.
$endgroup$
– Goal123
Jan 9 at 17:59




$begingroup$
@Keen $u$ is required to be one-one to apply change of variable, which need not be true here.
$endgroup$
– Goal123
Jan 9 at 17:59










1 Answer
1






active

oldest

votes


















2












$begingroup$

Take, for every $0 < t < 1$, $u(t)=e^{-1/t}$ and $u=0$ everywhere else. Take $varphi$ to be a smooth approximation of $1_{[0,1]}$, that is not lower than this function. On $[0,1]$, ${0 < u leq r}$ has measure $|ln{r}|^{-1}$.



Thus your integral is not lower than $int_{0<|u| leq epsilon/2}{|1-sin(pi/4)|^2} geq frac{c}{|ln(epsilon/2)|}$ where $c >0$ is a numeric constant. So the answer to your question is negative.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Why is the measure $|log r|^{-1}$?
    $endgroup$
    – Goal123
    Jan 10 at 9:36










  • $begingroup$
    $0 <u(t) leq r$ iff $0 < t < 1$ and $e^{-1/t} leq r$ and you can figure out the rest.
    $endgroup$
    – Mindlack
    Jan 10 at 9:43










  • $begingroup$
    But $e^{-1/t} leq r$ gives $ t ge -1/log r$ if $r >1$ and $t< -1/ log r$ if $r<1$.
    $endgroup$
    – Goal123
    Jan 10 at 9:55






  • 1




    $begingroup$
    But note that $r < 1$ always because $0 leq u leq e^{-1}$.
    $endgroup$
    – Mindlack
    Jan 10 at 10:02






  • 1




    $begingroup$
    When $r>0$ goes to $0$, then for any $a<0$, $|ln(r)| << r^a$, so this integral is $>>epsilon^{-a}$ for any $a <0$.
    $endgroup$
    – Mindlack
    Jan 10 at 10:24













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1 Answer
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active

oldest

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1 Answer
1






active

oldest

votes









active

oldest

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active

oldest

votes









2












$begingroup$

Take, for every $0 < t < 1$, $u(t)=e^{-1/t}$ and $u=0$ everywhere else. Take $varphi$ to be a smooth approximation of $1_{[0,1]}$, that is not lower than this function. On $[0,1]$, ${0 < u leq r}$ has measure $|ln{r}|^{-1}$.



Thus your integral is not lower than $int_{0<|u| leq epsilon/2}{|1-sin(pi/4)|^2} geq frac{c}{|ln(epsilon/2)|}$ where $c >0$ is a numeric constant. So the answer to your question is negative.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Why is the measure $|log r|^{-1}$?
    $endgroup$
    – Goal123
    Jan 10 at 9:36










  • $begingroup$
    $0 <u(t) leq r$ iff $0 < t < 1$ and $e^{-1/t} leq r$ and you can figure out the rest.
    $endgroup$
    – Mindlack
    Jan 10 at 9:43










  • $begingroup$
    But $e^{-1/t} leq r$ gives $ t ge -1/log r$ if $r >1$ and $t< -1/ log r$ if $r<1$.
    $endgroup$
    – Goal123
    Jan 10 at 9:55






  • 1




    $begingroup$
    But note that $r < 1$ always because $0 leq u leq e^{-1}$.
    $endgroup$
    – Mindlack
    Jan 10 at 10:02






  • 1




    $begingroup$
    When $r>0$ goes to $0$, then for any $a<0$, $|ln(r)| << r^a$, so this integral is $>>epsilon^{-a}$ for any $a <0$.
    $endgroup$
    – Mindlack
    Jan 10 at 10:24


















2












$begingroup$

Take, for every $0 < t < 1$, $u(t)=e^{-1/t}$ and $u=0$ everywhere else. Take $varphi$ to be a smooth approximation of $1_{[0,1]}$, that is not lower than this function. On $[0,1]$, ${0 < u leq r}$ has measure $|ln{r}|^{-1}$.



Thus your integral is not lower than $int_{0<|u| leq epsilon/2}{|1-sin(pi/4)|^2} geq frac{c}{|ln(epsilon/2)|}$ where $c >0$ is a numeric constant. So the answer to your question is negative.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Why is the measure $|log r|^{-1}$?
    $endgroup$
    – Goal123
    Jan 10 at 9:36










  • $begingroup$
    $0 <u(t) leq r$ iff $0 < t < 1$ and $e^{-1/t} leq r$ and you can figure out the rest.
    $endgroup$
    – Mindlack
    Jan 10 at 9:43










  • $begingroup$
    But $e^{-1/t} leq r$ gives $ t ge -1/log r$ if $r >1$ and $t< -1/ log r$ if $r<1$.
    $endgroup$
    – Goal123
    Jan 10 at 9:55






  • 1




    $begingroup$
    But note that $r < 1$ always because $0 leq u leq e^{-1}$.
    $endgroup$
    – Mindlack
    Jan 10 at 10:02






  • 1




    $begingroup$
    When $r>0$ goes to $0$, then for any $a<0$, $|ln(r)| << r^a$, so this integral is $>>epsilon^{-a}$ for any $a <0$.
    $endgroup$
    – Mindlack
    Jan 10 at 10:24
















2












2








2





$begingroup$

Take, for every $0 < t < 1$, $u(t)=e^{-1/t}$ and $u=0$ everywhere else. Take $varphi$ to be a smooth approximation of $1_{[0,1]}$, that is not lower than this function. On $[0,1]$, ${0 < u leq r}$ has measure $|ln{r}|^{-1}$.



Thus your integral is not lower than $int_{0<|u| leq epsilon/2}{|1-sin(pi/4)|^2} geq frac{c}{|ln(epsilon/2)|}$ where $c >0$ is a numeric constant. So the answer to your question is negative.






share|cite|improve this answer









$endgroup$



Take, for every $0 < t < 1$, $u(t)=e^{-1/t}$ and $u=0$ everywhere else. Take $varphi$ to be a smooth approximation of $1_{[0,1]}$, that is not lower than this function. On $[0,1]$, ${0 < u leq r}$ has measure $|ln{r}|^{-1}$.



Thus your integral is not lower than $int_{0<|u| leq epsilon/2}{|1-sin(pi/4)|^2} geq frac{c}{|ln(epsilon/2)|}$ where $c >0$ is a numeric constant. So the answer to your question is negative.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Jan 9 at 23:21









MindlackMindlack

4,750210




4,750210












  • $begingroup$
    Why is the measure $|log r|^{-1}$?
    $endgroup$
    – Goal123
    Jan 10 at 9:36










  • $begingroup$
    $0 <u(t) leq r$ iff $0 < t < 1$ and $e^{-1/t} leq r$ and you can figure out the rest.
    $endgroup$
    – Mindlack
    Jan 10 at 9:43










  • $begingroup$
    But $e^{-1/t} leq r$ gives $ t ge -1/log r$ if $r >1$ and $t< -1/ log r$ if $r<1$.
    $endgroup$
    – Goal123
    Jan 10 at 9:55






  • 1




    $begingroup$
    But note that $r < 1$ always because $0 leq u leq e^{-1}$.
    $endgroup$
    – Mindlack
    Jan 10 at 10:02






  • 1




    $begingroup$
    When $r>0$ goes to $0$, then for any $a<0$, $|ln(r)| << r^a$, so this integral is $>>epsilon^{-a}$ for any $a <0$.
    $endgroup$
    – Mindlack
    Jan 10 at 10:24




















  • $begingroup$
    Why is the measure $|log r|^{-1}$?
    $endgroup$
    – Goal123
    Jan 10 at 9:36










  • $begingroup$
    $0 <u(t) leq r$ iff $0 < t < 1$ and $e^{-1/t} leq r$ and you can figure out the rest.
    $endgroup$
    – Mindlack
    Jan 10 at 9:43










  • $begingroup$
    But $e^{-1/t} leq r$ gives $ t ge -1/log r$ if $r >1$ and $t< -1/ log r$ if $r<1$.
    $endgroup$
    – Goal123
    Jan 10 at 9:55






  • 1




    $begingroup$
    But note that $r < 1$ always because $0 leq u leq e^{-1}$.
    $endgroup$
    – Mindlack
    Jan 10 at 10:02






  • 1




    $begingroup$
    When $r>0$ goes to $0$, then for any $a<0$, $|ln(r)| << r^a$, so this integral is $>>epsilon^{-a}$ for any $a <0$.
    $endgroup$
    – Mindlack
    Jan 10 at 10:24


















$begingroup$
Why is the measure $|log r|^{-1}$?
$endgroup$
– Goal123
Jan 10 at 9:36




$begingroup$
Why is the measure $|log r|^{-1}$?
$endgroup$
– Goal123
Jan 10 at 9:36












$begingroup$
$0 <u(t) leq r$ iff $0 < t < 1$ and $e^{-1/t} leq r$ and you can figure out the rest.
$endgroup$
– Mindlack
Jan 10 at 9:43




$begingroup$
$0 <u(t) leq r$ iff $0 < t < 1$ and $e^{-1/t} leq r$ and you can figure out the rest.
$endgroup$
– Mindlack
Jan 10 at 9:43












$begingroup$
But $e^{-1/t} leq r$ gives $ t ge -1/log r$ if $r >1$ and $t< -1/ log r$ if $r<1$.
$endgroup$
– Goal123
Jan 10 at 9:55




$begingroup$
But $e^{-1/t} leq r$ gives $ t ge -1/log r$ if $r >1$ and $t< -1/ log r$ if $r<1$.
$endgroup$
– Goal123
Jan 10 at 9:55




1




1




$begingroup$
But note that $r < 1$ always because $0 leq u leq e^{-1}$.
$endgroup$
– Mindlack
Jan 10 at 10:02




$begingroup$
But note that $r < 1$ always because $0 leq u leq e^{-1}$.
$endgroup$
– Mindlack
Jan 10 at 10:02




1




1




$begingroup$
When $r>0$ goes to $0$, then for any $a<0$, $|ln(r)| << r^a$, so this integral is $>>epsilon^{-a}$ for any $a <0$.
$endgroup$
– Mindlack
Jan 10 at 10:24






$begingroup$
When $r>0$ goes to $0$, then for any $a<0$, $|ln(r)| << r^a$, so this integral is $>>epsilon^{-a}$ for any $a <0$.
$endgroup$
– Mindlack
Jan 10 at 10:24




















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