Convergence of $frac{cos(x)}{nsin(x)}$ an n$rightarrow infty$











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So I have this as my homework in Measure Theory and I don't know how to prove it. The question is the following:
On the domain $Omega = (0, pi)times {rm I!R}$ we have a function defined as:
begin{equation}
phi_{n}(x,y) = frac{cos(x)}{nsin(y)} qquad if,,yin {rm I!R}setminus {kpi | kin {rm I!N} } \ phi_{n}(x,y)=n^2 ,,,,,,qquad if,,yin {kpi | kin {rm I!N} }.
end{equation}

Now I need to prove that the sequence
begin{equation}
{int_{Omega}|phi_{n}(x,y)|dxdy}
end{equation}



is convergent almost everywhere. I know that since ${ kpi | kin {rm I!N}}$ is just a set of disjoint points, its Lebesgue measure is $0$, hence, we don't care about them. I was specifically told that I should use Fatou's lemma which we can apply due to the absolute sign. Thus, since we have sequence of positive functions we have
begin{equation}
int_{Omega}lim inf |phi_{n}| leq lim inf int_{Omega}|phi_{n}|.
end{equation}

Also, we have the inverse inequality for the $lim sup$ as well i.e.
begin{equation}
lim sup int_{Omega}|phi_{n}| leq int_{Omega}lim sup |phi_{n}|.
end{equation}

Then, if the $lim inf$ and $lim sup$ coincide, we have that the sequence is actually converges and we get that
begin{equation}
int_{Omega}lim inf |phi_{n}| = lim int_{Omega} |phi_{n}| = int_{Omega}| lim sup |phi_{n}|.
end{equation}

However, I am stuguling with how to use the properties of $phi_{n}$ to show that $lim inf = lim sup$. Any help would be appriciated.










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  • Look at the expression for $phi_n(x,y)$. What does it converge to almost everywhere?
    – T_M
    Nov 19 at 20:56










  • I know that it converges to 0, but I assumed that I should not know that, and by just using some properties of sin and cos we can prove that limsup and liminf are the same.
    – qake4
    Nov 19 at 20:59










  • Why did you assume that? The pointwise a.e. limit doesn't really have anything to do with sin and cos, that's kind of the "trick" with problems like this. At a.e. point $(x,y)$ its just a sequence of the form $const.times 1/n$
    – T_M
    Nov 19 at 21:02










  • So then I should just show that liminf = 0 and limsup = 0, if yes then please post it as an answer and when I am done with the proof (probably tomorrow), then I accept as an answer.
    – qake4
    Nov 19 at 21:07















up vote
1
down vote

favorite












So I have this as my homework in Measure Theory and I don't know how to prove it. The question is the following:
On the domain $Omega = (0, pi)times {rm I!R}$ we have a function defined as:
begin{equation}
phi_{n}(x,y) = frac{cos(x)}{nsin(y)} qquad if,,yin {rm I!R}setminus {kpi | kin {rm I!N} } \ phi_{n}(x,y)=n^2 ,,,,,,qquad if,,yin {kpi | kin {rm I!N} }.
end{equation}

Now I need to prove that the sequence
begin{equation}
{int_{Omega}|phi_{n}(x,y)|dxdy}
end{equation}



is convergent almost everywhere. I know that since ${ kpi | kin {rm I!N}}$ is just a set of disjoint points, its Lebesgue measure is $0$, hence, we don't care about them. I was specifically told that I should use Fatou's lemma which we can apply due to the absolute sign. Thus, since we have sequence of positive functions we have
begin{equation}
int_{Omega}lim inf |phi_{n}| leq lim inf int_{Omega}|phi_{n}|.
end{equation}

Also, we have the inverse inequality for the $lim sup$ as well i.e.
begin{equation}
lim sup int_{Omega}|phi_{n}| leq int_{Omega}lim sup |phi_{n}|.
end{equation}

Then, if the $lim inf$ and $lim sup$ coincide, we have that the sequence is actually converges and we get that
begin{equation}
int_{Omega}lim inf |phi_{n}| = lim int_{Omega} |phi_{n}| = int_{Omega}| lim sup |phi_{n}|.
end{equation}

However, I am stuguling with how to use the properties of $phi_{n}$ to show that $lim inf = lim sup$. Any help would be appriciated.










share|cite|improve this question






















  • Look at the expression for $phi_n(x,y)$. What does it converge to almost everywhere?
    – T_M
    Nov 19 at 20:56










  • I know that it converges to 0, but I assumed that I should not know that, and by just using some properties of sin and cos we can prove that limsup and liminf are the same.
    – qake4
    Nov 19 at 20:59










  • Why did you assume that? The pointwise a.e. limit doesn't really have anything to do with sin and cos, that's kind of the "trick" with problems like this. At a.e. point $(x,y)$ its just a sequence of the form $const.times 1/n$
    – T_M
    Nov 19 at 21:02










  • So then I should just show that liminf = 0 and limsup = 0, if yes then please post it as an answer and when I am done with the proof (probably tomorrow), then I accept as an answer.
    – qake4
    Nov 19 at 21:07













up vote
1
down vote

favorite









up vote
1
down vote

favorite











So I have this as my homework in Measure Theory and I don't know how to prove it. The question is the following:
On the domain $Omega = (0, pi)times {rm I!R}$ we have a function defined as:
begin{equation}
phi_{n}(x,y) = frac{cos(x)}{nsin(y)} qquad if,,yin {rm I!R}setminus {kpi | kin {rm I!N} } \ phi_{n}(x,y)=n^2 ,,,,,,qquad if,,yin {kpi | kin {rm I!N} }.
end{equation}

Now I need to prove that the sequence
begin{equation}
{int_{Omega}|phi_{n}(x,y)|dxdy}
end{equation}



is convergent almost everywhere. I know that since ${ kpi | kin {rm I!N}}$ is just a set of disjoint points, its Lebesgue measure is $0$, hence, we don't care about them. I was specifically told that I should use Fatou's lemma which we can apply due to the absolute sign. Thus, since we have sequence of positive functions we have
begin{equation}
int_{Omega}lim inf |phi_{n}| leq lim inf int_{Omega}|phi_{n}|.
end{equation}

Also, we have the inverse inequality for the $lim sup$ as well i.e.
begin{equation}
lim sup int_{Omega}|phi_{n}| leq int_{Omega}lim sup |phi_{n}|.
end{equation}

Then, if the $lim inf$ and $lim sup$ coincide, we have that the sequence is actually converges and we get that
begin{equation}
int_{Omega}lim inf |phi_{n}| = lim int_{Omega} |phi_{n}| = int_{Omega}| lim sup |phi_{n}|.
end{equation}

However, I am stuguling with how to use the properties of $phi_{n}$ to show that $lim inf = lim sup$. Any help would be appriciated.










share|cite|improve this question













So I have this as my homework in Measure Theory and I don't know how to prove it. The question is the following:
On the domain $Omega = (0, pi)times {rm I!R}$ we have a function defined as:
begin{equation}
phi_{n}(x,y) = frac{cos(x)}{nsin(y)} qquad if,,yin {rm I!R}setminus {kpi | kin {rm I!N} } \ phi_{n}(x,y)=n^2 ,,,,,,qquad if,,yin {kpi | kin {rm I!N} }.
end{equation}

Now I need to prove that the sequence
begin{equation}
{int_{Omega}|phi_{n}(x,y)|dxdy}
end{equation}



is convergent almost everywhere. I know that since ${ kpi | kin {rm I!N}}$ is just a set of disjoint points, its Lebesgue measure is $0$, hence, we don't care about them. I was specifically told that I should use Fatou's lemma which we can apply due to the absolute sign. Thus, since we have sequence of positive functions we have
begin{equation}
int_{Omega}lim inf |phi_{n}| leq lim inf int_{Omega}|phi_{n}|.
end{equation}

Also, we have the inverse inequality for the $lim sup$ as well i.e.
begin{equation}
lim sup int_{Omega}|phi_{n}| leq int_{Omega}lim sup |phi_{n}|.
end{equation}

Then, if the $lim inf$ and $lim sup$ coincide, we have that the sequence is actually converges and we get that
begin{equation}
int_{Omega}lim inf |phi_{n}| = lim int_{Omega} |phi_{n}| = int_{Omega}| lim sup |phi_{n}|.
end{equation}

However, I am stuguling with how to use the properties of $phi_{n}$ to show that $lim inf = lim sup$. Any help would be appriciated.







measure-theory lebesgue-measure






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asked Nov 19 at 20:52









qake4

253




253












  • Look at the expression for $phi_n(x,y)$. What does it converge to almost everywhere?
    – T_M
    Nov 19 at 20:56










  • I know that it converges to 0, but I assumed that I should not know that, and by just using some properties of sin and cos we can prove that limsup and liminf are the same.
    – qake4
    Nov 19 at 20:59










  • Why did you assume that? The pointwise a.e. limit doesn't really have anything to do with sin and cos, that's kind of the "trick" with problems like this. At a.e. point $(x,y)$ its just a sequence of the form $const.times 1/n$
    – T_M
    Nov 19 at 21:02










  • So then I should just show that liminf = 0 and limsup = 0, if yes then please post it as an answer and when I am done with the proof (probably tomorrow), then I accept as an answer.
    – qake4
    Nov 19 at 21:07


















  • Look at the expression for $phi_n(x,y)$. What does it converge to almost everywhere?
    – T_M
    Nov 19 at 20:56










  • I know that it converges to 0, but I assumed that I should not know that, and by just using some properties of sin and cos we can prove that limsup and liminf are the same.
    – qake4
    Nov 19 at 20:59










  • Why did you assume that? The pointwise a.e. limit doesn't really have anything to do with sin and cos, that's kind of the "trick" with problems like this. At a.e. point $(x,y)$ its just a sequence of the form $const.times 1/n$
    – T_M
    Nov 19 at 21:02










  • So then I should just show that liminf = 0 and limsup = 0, if yes then please post it as an answer and when I am done with the proof (probably tomorrow), then I accept as an answer.
    – qake4
    Nov 19 at 21:07
















Look at the expression for $phi_n(x,y)$. What does it converge to almost everywhere?
– T_M
Nov 19 at 20:56




Look at the expression for $phi_n(x,y)$. What does it converge to almost everywhere?
– T_M
Nov 19 at 20:56












I know that it converges to 0, but I assumed that I should not know that, and by just using some properties of sin and cos we can prove that limsup and liminf are the same.
– qake4
Nov 19 at 20:59




I know that it converges to 0, but I assumed that I should not know that, and by just using some properties of sin and cos we can prove that limsup and liminf are the same.
– qake4
Nov 19 at 20:59












Why did you assume that? The pointwise a.e. limit doesn't really have anything to do with sin and cos, that's kind of the "trick" with problems like this. At a.e. point $(x,y)$ its just a sequence of the form $const.times 1/n$
– T_M
Nov 19 at 21:02




Why did you assume that? The pointwise a.e. limit doesn't really have anything to do with sin and cos, that's kind of the "trick" with problems like this. At a.e. point $(x,y)$ its just a sequence of the form $const.times 1/n$
– T_M
Nov 19 at 21:02












So then I should just show that liminf = 0 and limsup = 0, if yes then please post it as an answer and when I am done with the proof (probably tomorrow), then I accept as an answer.
– qake4
Nov 19 at 21:07




So then I should just show that liminf = 0 and limsup = 0, if yes then please post it as an answer and when I am done with the proof (probably tomorrow), then I accept as an answer.
– qake4
Nov 19 at 21:07










1 Answer
1






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1
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Let $Y={kpi : kinmathbb N}$ then $|Y|=0$ so by Fubini theorem
$$
int_{Omega} |phi_n(x, y)|dxdy = frac 1nint_{Omegasetminus Y}leftlvertfrac{cos x}{sin y}rightrvert dxdy=frac 1nint^pi_0lvertcos xrvert dxint_{mathbb Rsetminus Y}leftlvertfrac{1}{sin y}rightrvert dy=
$$



Now
$$
int_{mathbb Rsetminus Y}leftlvertfrac{1}{sin y}rightrvert dy=sum^{+infty}_{i=-infty}int^{(i+1)pi}_{ipi}leftlvertfrac{1}{sin y}rightrvert dy=sum^{+infty}_{i=-infty}int^{pi}_{0}leftlvertfrac{1}{sin y}rightrvert dy=+infty
$$



and doesn't converge






share|cite|improve this answer





















  • That is true, $int |1/sin(y)|$ $rightarrow infty$ but as was pointed out by @T_M, we can consider $int |1/sin(y)|$ as a constant for all $y in Rsetminus Y$ which is less than $infty$ and as n $rightarrow infty$ we indeed get that $int |phi_{n}(x,y)|$ converges to 0, don't we?
    – qake4
    Nov 19 at 23:38












  • The integral doesn't converge for all $n$, if $frac{1}{lvertsin yrvert}$ is integrated on a set smaller than $mathbb R$ (like compact set) the integral may be considered as a finite constant
    – P De Donato
    Nov 20 at 0:40











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Let $Y={kpi : kinmathbb N}$ then $|Y|=0$ so by Fubini theorem
$$
int_{Omega} |phi_n(x, y)|dxdy = frac 1nint_{Omegasetminus Y}leftlvertfrac{cos x}{sin y}rightrvert dxdy=frac 1nint^pi_0lvertcos xrvert dxint_{mathbb Rsetminus Y}leftlvertfrac{1}{sin y}rightrvert dy=
$$



Now
$$
int_{mathbb Rsetminus Y}leftlvertfrac{1}{sin y}rightrvert dy=sum^{+infty}_{i=-infty}int^{(i+1)pi}_{ipi}leftlvertfrac{1}{sin y}rightrvert dy=sum^{+infty}_{i=-infty}int^{pi}_{0}leftlvertfrac{1}{sin y}rightrvert dy=+infty
$$



and doesn't converge






share|cite|improve this answer





















  • That is true, $int |1/sin(y)|$ $rightarrow infty$ but as was pointed out by @T_M, we can consider $int |1/sin(y)|$ as a constant for all $y in Rsetminus Y$ which is less than $infty$ and as n $rightarrow infty$ we indeed get that $int |phi_{n}(x,y)|$ converges to 0, don't we?
    – qake4
    Nov 19 at 23:38












  • The integral doesn't converge for all $n$, if $frac{1}{lvertsin yrvert}$ is integrated on a set smaller than $mathbb R$ (like compact set) the integral may be considered as a finite constant
    – P De Donato
    Nov 20 at 0:40















up vote
1
down vote













Let $Y={kpi : kinmathbb N}$ then $|Y|=0$ so by Fubini theorem
$$
int_{Omega} |phi_n(x, y)|dxdy = frac 1nint_{Omegasetminus Y}leftlvertfrac{cos x}{sin y}rightrvert dxdy=frac 1nint^pi_0lvertcos xrvert dxint_{mathbb Rsetminus Y}leftlvertfrac{1}{sin y}rightrvert dy=
$$



Now
$$
int_{mathbb Rsetminus Y}leftlvertfrac{1}{sin y}rightrvert dy=sum^{+infty}_{i=-infty}int^{(i+1)pi}_{ipi}leftlvertfrac{1}{sin y}rightrvert dy=sum^{+infty}_{i=-infty}int^{pi}_{0}leftlvertfrac{1}{sin y}rightrvert dy=+infty
$$



and doesn't converge






share|cite|improve this answer





















  • That is true, $int |1/sin(y)|$ $rightarrow infty$ but as was pointed out by @T_M, we can consider $int |1/sin(y)|$ as a constant for all $y in Rsetminus Y$ which is less than $infty$ and as n $rightarrow infty$ we indeed get that $int |phi_{n}(x,y)|$ converges to 0, don't we?
    – qake4
    Nov 19 at 23:38












  • The integral doesn't converge for all $n$, if $frac{1}{lvertsin yrvert}$ is integrated on a set smaller than $mathbb R$ (like compact set) the integral may be considered as a finite constant
    – P De Donato
    Nov 20 at 0:40













up vote
1
down vote










up vote
1
down vote









Let $Y={kpi : kinmathbb N}$ then $|Y|=0$ so by Fubini theorem
$$
int_{Omega} |phi_n(x, y)|dxdy = frac 1nint_{Omegasetminus Y}leftlvertfrac{cos x}{sin y}rightrvert dxdy=frac 1nint^pi_0lvertcos xrvert dxint_{mathbb Rsetminus Y}leftlvertfrac{1}{sin y}rightrvert dy=
$$



Now
$$
int_{mathbb Rsetminus Y}leftlvertfrac{1}{sin y}rightrvert dy=sum^{+infty}_{i=-infty}int^{(i+1)pi}_{ipi}leftlvertfrac{1}{sin y}rightrvert dy=sum^{+infty}_{i=-infty}int^{pi}_{0}leftlvertfrac{1}{sin y}rightrvert dy=+infty
$$



and doesn't converge






share|cite|improve this answer












Let $Y={kpi : kinmathbb N}$ then $|Y|=0$ so by Fubini theorem
$$
int_{Omega} |phi_n(x, y)|dxdy = frac 1nint_{Omegasetminus Y}leftlvertfrac{cos x}{sin y}rightrvert dxdy=frac 1nint^pi_0lvertcos xrvert dxint_{mathbb Rsetminus Y}leftlvertfrac{1}{sin y}rightrvert dy=
$$



Now
$$
int_{mathbb Rsetminus Y}leftlvertfrac{1}{sin y}rightrvert dy=sum^{+infty}_{i=-infty}int^{(i+1)pi}_{ipi}leftlvertfrac{1}{sin y}rightrvert dy=sum^{+infty}_{i=-infty}int^{pi}_{0}leftlvertfrac{1}{sin y}rightrvert dy=+infty
$$



and doesn't converge







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Nov 19 at 21:53









P De Donato

3317




3317












  • That is true, $int |1/sin(y)|$ $rightarrow infty$ but as was pointed out by @T_M, we can consider $int |1/sin(y)|$ as a constant for all $y in Rsetminus Y$ which is less than $infty$ and as n $rightarrow infty$ we indeed get that $int |phi_{n}(x,y)|$ converges to 0, don't we?
    – qake4
    Nov 19 at 23:38












  • The integral doesn't converge for all $n$, if $frac{1}{lvertsin yrvert}$ is integrated on a set smaller than $mathbb R$ (like compact set) the integral may be considered as a finite constant
    – P De Donato
    Nov 20 at 0:40


















  • That is true, $int |1/sin(y)|$ $rightarrow infty$ but as was pointed out by @T_M, we can consider $int |1/sin(y)|$ as a constant for all $y in Rsetminus Y$ which is less than $infty$ and as n $rightarrow infty$ we indeed get that $int |phi_{n}(x,y)|$ converges to 0, don't we?
    – qake4
    Nov 19 at 23:38












  • The integral doesn't converge for all $n$, if $frac{1}{lvertsin yrvert}$ is integrated on a set smaller than $mathbb R$ (like compact set) the integral may be considered as a finite constant
    – P De Donato
    Nov 20 at 0:40
















That is true, $int |1/sin(y)|$ $rightarrow infty$ but as was pointed out by @T_M, we can consider $int |1/sin(y)|$ as a constant for all $y in Rsetminus Y$ which is less than $infty$ and as n $rightarrow infty$ we indeed get that $int |phi_{n}(x,y)|$ converges to 0, don't we?
– qake4
Nov 19 at 23:38






That is true, $int |1/sin(y)|$ $rightarrow infty$ but as was pointed out by @T_M, we can consider $int |1/sin(y)|$ as a constant for all $y in Rsetminus Y$ which is less than $infty$ and as n $rightarrow infty$ we indeed get that $int |phi_{n}(x,y)|$ converges to 0, don't we?
– qake4
Nov 19 at 23:38














The integral doesn't converge for all $n$, if $frac{1}{lvertsin yrvert}$ is integrated on a set smaller than $mathbb R$ (like compact set) the integral may be considered as a finite constant
– P De Donato
Nov 20 at 0:40




The integral doesn't converge for all $n$, if $frac{1}{lvertsin yrvert}$ is integrated on a set smaller than $mathbb R$ (like compact set) the integral may be considered as a finite constant
– P De Donato
Nov 20 at 0:40


















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