Compute $frac{1}{2pi}int_0^{2pi}f(t)g(t)dt$ where $g(t)= sumlimits_{n=1}^infty frac{(-1)^n}{n}sin(nt)$












3












$begingroup$



Fourier analysis, compute integral $dfrac{1}{2pi}int_0^{2pi}f(t)g(t)dt$ where $g(t)= sum_{n=1}^infty dfrac{(-1)^n}{n}sin(nt)$ and $f(t)=cos(t)+7sin(2t)$.




I'm having slight problems solving this integral, can we use alternating series test to say that the sum converges in some way? It feels like the sum might converge to $0$ and that we in that case will get an integal that is just $0$, but that feels counterintuitive so I'm unsure. Is there some theorem that might help that I'm missing?










share|cite|improve this question











$endgroup$












  • $begingroup$
    What is $f(t)$? And can't you use the closed form of $g(t)$?
    $endgroup$
    – metamorphy
    Dec 18 '18 at 13:06










  • $begingroup$
    Thanks! forgot to add $f(t)$ have now added it.
    $endgroup$
    – L200123
    Dec 18 '18 at 13:11






  • 1




    $begingroup$
    I don't know if you are supposed to recognize this, but I think $g(t)$ is the same as $-t/2$ (which makes this just an integration by parts problem).
    $endgroup$
    – Rellek
    Dec 18 '18 at 13:32










  • $begingroup$
    Remember that for $|x|<pi$, $$x^2=frac{pi^2}3+4sum_{ngeq1}frac{(-1)^n}{n^2}cos nx$$ So then you take $d/dx$ on both sides to get $$-2x=4sum_{ngeq1}frac{(-1)^n}{n}sin nx$$ so we have that $$g(x)=-frac{x}2$$
    $endgroup$
    – clathratus
    Dec 18 '18 at 18:34








  • 1




    $begingroup$
    @Rellek here $g(t)=-t/2$ when $tin[-pi,pi]$, but he is integrating in $[0,2pi]$, in this set $$g(t)=begin{cases}-t/2,&tin[0,pi)\pi-t/2,&tin(pi,2pi]\0,&t=piend{cases}$$
    $endgroup$
    – Masacroso
    Dec 18 '18 at 18:41
















3












$begingroup$



Fourier analysis, compute integral $dfrac{1}{2pi}int_0^{2pi}f(t)g(t)dt$ where $g(t)= sum_{n=1}^infty dfrac{(-1)^n}{n}sin(nt)$ and $f(t)=cos(t)+7sin(2t)$.




I'm having slight problems solving this integral, can we use alternating series test to say that the sum converges in some way? It feels like the sum might converge to $0$ and that we in that case will get an integal that is just $0$, but that feels counterintuitive so I'm unsure. Is there some theorem that might help that I'm missing?










share|cite|improve this question











$endgroup$












  • $begingroup$
    What is $f(t)$? And can't you use the closed form of $g(t)$?
    $endgroup$
    – metamorphy
    Dec 18 '18 at 13:06










  • $begingroup$
    Thanks! forgot to add $f(t)$ have now added it.
    $endgroup$
    – L200123
    Dec 18 '18 at 13:11






  • 1




    $begingroup$
    I don't know if you are supposed to recognize this, but I think $g(t)$ is the same as $-t/2$ (which makes this just an integration by parts problem).
    $endgroup$
    – Rellek
    Dec 18 '18 at 13:32










  • $begingroup$
    Remember that for $|x|<pi$, $$x^2=frac{pi^2}3+4sum_{ngeq1}frac{(-1)^n}{n^2}cos nx$$ So then you take $d/dx$ on both sides to get $$-2x=4sum_{ngeq1}frac{(-1)^n}{n}sin nx$$ so we have that $$g(x)=-frac{x}2$$
    $endgroup$
    – clathratus
    Dec 18 '18 at 18:34








  • 1




    $begingroup$
    @Rellek here $g(t)=-t/2$ when $tin[-pi,pi]$, but he is integrating in $[0,2pi]$, in this set $$g(t)=begin{cases}-t/2,&tin[0,pi)\pi-t/2,&tin(pi,2pi]\0,&t=piend{cases}$$
    $endgroup$
    – Masacroso
    Dec 18 '18 at 18:41














3












3








3





$begingroup$



Fourier analysis, compute integral $dfrac{1}{2pi}int_0^{2pi}f(t)g(t)dt$ where $g(t)= sum_{n=1}^infty dfrac{(-1)^n}{n}sin(nt)$ and $f(t)=cos(t)+7sin(2t)$.




I'm having slight problems solving this integral, can we use alternating series test to say that the sum converges in some way? It feels like the sum might converge to $0$ and that we in that case will get an integal that is just $0$, but that feels counterintuitive so I'm unsure. Is there some theorem that might help that I'm missing?










share|cite|improve this question











$endgroup$





Fourier analysis, compute integral $dfrac{1}{2pi}int_0^{2pi}f(t)g(t)dt$ where $g(t)= sum_{n=1}^infty dfrac{(-1)^n}{n}sin(nt)$ and $f(t)=cos(t)+7sin(2t)$.




I'm having slight problems solving this integral, can we use alternating series test to say that the sum converges in some way? It feels like the sum might converge to $0$ and that we in that case will get an integal that is just $0$, but that feels counterintuitive so I'm unsure. Is there some theorem that might help that I'm missing?







real-analysis integration fourier-analysis






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 18 '18 at 21:34









Did

248k23224462




248k23224462










asked Dec 18 '18 at 13:00









L200123L200123

855




855












  • $begingroup$
    What is $f(t)$? And can't you use the closed form of $g(t)$?
    $endgroup$
    – metamorphy
    Dec 18 '18 at 13:06










  • $begingroup$
    Thanks! forgot to add $f(t)$ have now added it.
    $endgroup$
    – L200123
    Dec 18 '18 at 13:11






  • 1




    $begingroup$
    I don't know if you are supposed to recognize this, but I think $g(t)$ is the same as $-t/2$ (which makes this just an integration by parts problem).
    $endgroup$
    – Rellek
    Dec 18 '18 at 13:32










  • $begingroup$
    Remember that for $|x|<pi$, $$x^2=frac{pi^2}3+4sum_{ngeq1}frac{(-1)^n}{n^2}cos nx$$ So then you take $d/dx$ on both sides to get $$-2x=4sum_{ngeq1}frac{(-1)^n}{n}sin nx$$ so we have that $$g(x)=-frac{x}2$$
    $endgroup$
    – clathratus
    Dec 18 '18 at 18:34








  • 1




    $begingroup$
    @Rellek here $g(t)=-t/2$ when $tin[-pi,pi]$, but he is integrating in $[0,2pi]$, in this set $$g(t)=begin{cases}-t/2,&tin[0,pi)\pi-t/2,&tin(pi,2pi]\0,&t=piend{cases}$$
    $endgroup$
    – Masacroso
    Dec 18 '18 at 18:41


















  • $begingroup$
    What is $f(t)$? And can't you use the closed form of $g(t)$?
    $endgroup$
    – metamorphy
    Dec 18 '18 at 13:06










  • $begingroup$
    Thanks! forgot to add $f(t)$ have now added it.
    $endgroup$
    – L200123
    Dec 18 '18 at 13:11






  • 1




    $begingroup$
    I don't know if you are supposed to recognize this, but I think $g(t)$ is the same as $-t/2$ (which makes this just an integration by parts problem).
    $endgroup$
    – Rellek
    Dec 18 '18 at 13:32










  • $begingroup$
    Remember that for $|x|<pi$, $$x^2=frac{pi^2}3+4sum_{ngeq1}frac{(-1)^n}{n^2}cos nx$$ So then you take $d/dx$ on both sides to get $$-2x=4sum_{ngeq1}frac{(-1)^n}{n}sin nx$$ so we have that $$g(x)=-frac{x}2$$
    $endgroup$
    – clathratus
    Dec 18 '18 at 18:34








  • 1




    $begingroup$
    @Rellek here $g(t)=-t/2$ when $tin[-pi,pi]$, but he is integrating in $[0,2pi]$, in this set $$g(t)=begin{cases}-t/2,&tin[0,pi)\pi-t/2,&tin(pi,2pi]\0,&t=piend{cases}$$
    $endgroup$
    – Masacroso
    Dec 18 '18 at 18:41
















$begingroup$
What is $f(t)$? And can't you use the closed form of $g(t)$?
$endgroup$
– metamorphy
Dec 18 '18 at 13:06




$begingroup$
What is $f(t)$? And can't you use the closed form of $g(t)$?
$endgroup$
– metamorphy
Dec 18 '18 at 13:06












$begingroup$
Thanks! forgot to add $f(t)$ have now added it.
$endgroup$
– L200123
Dec 18 '18 at 13:11




$begingroup$
Thanks! forgot to add $f(t)$ have now added it.
$endgroup$
– L200123
Dec 18 '18 at 13:11




1




1




$begingroup$
I don't know if you are supposed to recognize this, but I think $g(t)$ is the same as $-t/2$ (which makes this just an integration by parts problem).
$endgroup$
– Rellek
Dec 18 '18 at 13:32




$begingroup$
I don't know if you are supposed to recognize this, but I think $g(t)$ is the same as $-t/2$ (which makes this just an integration by parts problem).
$endgroup$
– Rellek
Dec 18 '18 at 13:32












$begingroup$
Remember that for $|x|<pi$, $$x^2=frac{pi^2}3+4sum_{ngeq1}frac{(-1)^n}{n^2}cos nx$$ So then you take $d/dx$ on both sides to get $$-2x=4sum_{ngeq1}frac{(-1)^n}{n}sin nx$$ so we have that $$g(x)=-frac{x}2$$
$endgroup$
– clathratus
Dec 18 '18 at 18:34






$begingroup$
Remember that for $|x|<pi$, $$x^2=frac{pi^2}3+4sum_{ngeq1}frac{(-1)^n}{n^2}cos nx$$ So then you take $d/dx$ on both sides to get $$-2x=4sum_{ngeq1}frac{(-1)^n}{n}sin nx$$ so we have that $$g(x)=-frac{x}2$$
$endgroup$
– clathratus
Dec 18 '18 at 18:34






1




1




$begingroup$
@Rellek here $g(t)=-t/2$ when $tin[-pi,pi]$, but he is integrating in $[0,2pi]$, in this set $$g(t)=begin{cases}-t/2,&tin[0,pi)\pi-t/2,&tin(pi,2pi]\0,&t=piend{cases}$$
$endgroup$
– Masacroso
Dec 18 '18 at 18:41




$begingroup$
@Rellek here $g(t)=-t/2$ when $tin[-pi,pi]$, but he is integrating in $[0,2pi]$, in this set $$g(t)=begin{cases}-t/2,&tin[0,pi)\pi-t/2,&tin(pi,2pi]\0,&t=piend{cases}$$
$endgroup$
– Masacroso
Dec 18 '18 at 18:41










1 Answer
1






active

oldest

votes


















1












$begingroup$

The sum for $g$ converges in $L^2$ because $sum_{n=1}^{infty}frac{1}{n^2} < infty$ and because ${ s_n(x)=frac{sin(nt)}{sqrt{pi}} }_{n=1}^{infty}$ is an orthonormal subset of $L^2[0,2pi]$. That is,
$$
g_N(t) = sum_{n=1}^{N}frac{(-1)^n}{n}sin(nt)
$$

converges in $L^2$ to some $gin L^2$. Because of this,
$$
int_{0}^{2pi}g(t)f(t)dt=lim_{N}int_{0}^{2pi}g_N(t)f(t)dt \
= sum_{n=1}^{infty}frac{(-1)^n}{n}int_{0}^{2pi}sin(nt)f(t)dt \
= sum_{n=1}^{infty}frac{(-1)^n}{n}int_{0}^{2pi}sin(nt)(cos(t)+7sin(2t))dt.
$$



The functions ${ 1,cos(t),sin(t),cos(2t),sin(2t),cdots }$ are mutually orthogonal. So the above reduces to



$$
frac{7}{2}int_0^{2pi}sin^2(2t)dt=frac{7pi}{2}.
$$

Therefore,
$$
frac{1}{2pi}int_{0}^{2pi}f(t)g(t)dt = frac{7}{4}.
$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    it is not enough clear that, because $g_n$ converges in $L^2$, we can exchange sum and integral. I will say that we can do this because $fin L^2$ also and the inner product in $L^2(Bbb R)$ is bounded and hence continuous, so $$lim_{ntoinfty}(g_n|f)=(g| f)$$
    $endgroup$
    – Masacroso
    Dec 20 '18 at 15:14













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1 Answer
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1 Answer
1






active

oldest

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active

oldest

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active

oldest

votes









1












$begingroup$

The sum for $g$ converges in $L^2$ because $sum_{n=1}^{infty}frac{1}{n^2} < infty$ and because ${ s_n(x)=frac{sin(nt)}{sqrt{pi}} }_{n=1}^{infty}$ is an orthonormal subset of $L^2[0,2pi]$. That is,
$$
g_N(t) = sum_{n=1}^{N}frac{(-1)^n}{n}sin(nt)
$$

converges in $L^2$ to some $gin L^2$. Because of this,
$$
int_{0}^{2pi}g(t)f(t)dt=lim_{N}int_{0}^{2pi}g_N(t)f(t)dt \
= sum_{n=1}^{infty}frac{(-1)^n}{n}int_{0}^{2pi}sin(nt)f(t)dt \
= sum_{n=1}^{infty}frac{(-1)^n}{n}int_{0}^{2pi}sin(nt)(cos(t)+7sin(2t))dt.
$$



The functions ${ 1,cos(t),sin(t),cos(2t),sin(2t),cdots }$ are mutually orthogonal. So the above reduces to



$$
frac{7}{2}int_0^{2pi}sin^2(2t)dt=frac{7pi}{2}.
$$

Therefore,
$$
frac{1}{2pi}int_{0}^{2pi}f(t)g(t)dt = frac{7}{4}.
$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    it is not enough clear that, because $g_n$ converges in $L^2$, we can exchange sum and integral. I will say that we can do this because $fin L^2$ also and the inner product in $L^2(Bbb R)$ is bounded and hence continuous, so $$lim_{ntoinfty}(g_n|f)=(g| f)$$
    $endgroup$
    – Masacroso
    Dec 20 '18 at 15:14


















1












$begingroup$

The sum for $g$ converges in $L^2$ because $sum_{n=1}^{infty}frac{1}{n^2} < infty$ and because ${ s_n(x)=frac{sin(nt)}{sqrt{pi}} }_{n=1}^{infty}$ is an orthonormal subset of $L^2[0,2pi]$. That is,
$$
g_N(t) = sum_{n=1}^{N}frac{(-1)^n}{n}sin(nt)
$$

converges in $L^2$ to some $gin L^2$. Because of this,
$$
int_{0}^{2pi}g(t)f(t)dt=lim_{N}int_{0}^{2pi}g_N(t)f(t)dt \
= sum_{n=1}^{infty}frac{(-1)^n}{n}int_{0}^{2pi}sin(nt)f(t)dt \
= sum_{n=1}^{infty}frac{(-1)^n}{n}int_{0}^{2pi}sin(nt)(cos(t)+7sin(2t))dt.
$$



The functions ${ 1,cos(t),sin(t),cos(2t),sin(2t),cdots }$ are mutually orthogonal. So the above reduces to



$$
frac{7}{2}int_0^{2pi}sin^2(2t)dt=frac{7pi}{2}.
$$

Therefore,
$$
frac{1}{2pi}int_{0}^{2pi}f(t)g(t)dt = frac{7}{4}.
$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    it is not enough clear that, because $g_n$ converges in $L^2$, we can exchange sum and integral. I will say that we can do this because $fin L^2$ also and the inner product in $L^2(Bbb R)$ is bounded and hence continuous, so $$lim_{ntoinfty}(g_n|f)=(g| f)$$
    $endgroup$
    – Masacroso
    Dec 20 '18 at 15:14
















1












1








1





$begingroup$

The sum for $g$ converges in $L^2$ because $sum_{n=1}^{infty}frac{1}{n^2} < infty$ and because ${ s_n(x)=frac{sin(nt)}{sqrt{pi}} }_{n=1}^{infty}$ is an orthonormal subset of $L^2[0,2pi]$. That is,
$$
g_N(t) = sum_{n=1}^{N}frac{(-1)^n}{n}sin(nt)
$$

converges in $L^2$ to some $gin L^2$. Because of this,
$$
int_{0}^{2pi}g(t)f(t)dt=lim_{N}int_{0}^{2pi}g_N(t)f(t)dt \
= sum_{n=1}^{infty}frac{(-1)^n}{n}int_{0}^{2pi}sin(nt)f(t)dt \
= sum_{n=1}^{infty}frac{(-1)^n}{n}int_{0}^{2pi}sin(nt)(cos(t)+7sin(2t))dt.
$$



The functions ${ 1,cos(t),sin(t),cos(2t),sin(2t),cdots }$ are mutually orthogonal. So the above reduces to



$$
frac{7}{2}int_0^{2pi}sin^2(2t)dt=frac{7pi}{2}.
$$

Therefore,
$$
frac{1}{2pi}int_{0}^{2pi}f(t)g(t)dt = frac{7}{4}.
$$






share|cite|improve this answer









$endgroup$



The sum for $g$ converges in $L^2$ because $sum_{n=1}^{infty}frac{1}{n^2} < infty$ and because ${ s_n(x)=frac{sin(nt)}{sqrt{pi}} }_{n=1}^{infty}$ is an orthonormal subset of $L^2[0,2pi]$. That is,
$$
g_N(t) = sum_{n=1}^{N}frac{(-1)^n}{n}sin(nt)
$$

converges in $L^2$ to some $gin L^2$. Because of this,
$$
int_{0}^{2pi}g(t)f(t)dt=lim_{N}int_{0}^{2pi}g_N(t)f(t)dt \
= sum_{n=1}^{infty}frac{(-1)^n}{n}int_{0}^{2pi}sin(nt)f(t)dt \
= sum_{n=1}^{infty}frac{(-1)^n}{n}int_{0}^{2pi}sin(nt)(cos(t)+7sin(2t))dt.
$$



The functions ${ 1,cos(t),sin(t),cos(2t),sin(2t),cdots }$ are mutually orthogonal. So the above reduces to



$$
frac{7}{2}int_0^{2pi}sin^2(2t)dt=frac{7pi}{2}.
$$

Therefore,
$$
frac{1}{2pi}int_{0}^{2pi}f(t)g(t)dt = frac{7}{4}.
$$







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Dec 18 '18 at 17:10









DisintegratingByPartsDisintegratingByParts

59.4k42580




59.4k42580












  • $begingroup$
    it is not enough clear that, because $g_n$ converges in $L^2$, we can exchange sum and integral. I will say that we can do this because $fin L^2$ also and the inner product in $L^2(Bbb R)$ is bounded and hence continuous, so $$lim_{ntoinfty}(g_n|f)=(g| f)$$
    $endgroup$
    – Masacroso
    Dec 20 '18 at 15:14




















  • $begingroup$
    it is not enough clear that, because $g_n$ converges in $L^2$, we can exchange sum and integral. I will say that we can do this because $fin L^2$ also and the inner product in $L^2(Bbb R)$ is bounded and hence continuous, so $$lim_{ntoinfty}(g_n|f)=(g| f)$$
    $endgroup$
    – Masacroso
    Dec 20 '18 at 15:14


















$begingroup$
it is not enough clear that, because $g_n$ converges in $L^2$, we can exchange sum and integral. I will say that we can do this because $fin L^2$ also and the inner product in $L^2(Bbb R)$ is bounded and hence continuous, so $$lim_{ntoinfty}(g_n|f)=(g| f)$$
$endgroup$
– Masacroso
Dec 20 '18 at 15:14






$begingroup$
it is not enough clear that, because $g_n$ converges in $L^2$, we can exchange sum and integral. I will say that we can do this because $fin L^2$ also and the inner product in $L^2(Bbb R)$ is bounded and hence continuous, so $$lim_{ntoinfty}(g_n|f)=(g| f)$$
$endgroup$
– Masacroso
Dec 20 '18 at 15:14




















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