Create Fourier-Series of f(x) = x if 0 < x < Pi and 0 if Pi < x < 2*Pi












1












$begingroup$


I tried the following to create the Fourier-series of the function:



$$ f(x) = begin{cases} x & 0<x<pi \ 0 & pi < x < 2 pi end{cases}$$



This is what I tried:



$$a_0 = frac{1}{pi}intlimits_0^pi x dx = frac{1}{pi}cdotleft(frac{x^2}{2}|_0^piright) = frac{1}{pi}cdotfrac{pi^2}{2} = frac{pi^2}{2cdotpi} = frac{pi}{2}$$



Is that right?



$$a_n = frac{1}{pi}intlimits_0^pi x cdot cos(nx) dx = frac{1}{pi}cdotleft(frac{x cdot sin(nx)}{n} + frac{cos(nx)}{n^2}right)|_0^pi$$



Assuming that $sin(nx)$ is always $0$ and $cos(nx)$ is $(-1)^n$. Is that ok??



$$ = frac{1}{pi}cdotleft(frac{0}{n} + frac{(-1)^n}{n^2}right) = frac{1}{pi}cdotleft(frac{(-1)^n}{n^2}right) = frac{(-1)^n}{pi n^2}$$



That's already wrong I'm sure!



$$b_n = frac{1}{pi}intlimits_0^pi x sin(nx) dx = frac{1}{pi}cdotleft(frac{x cdot cos(nx)}{n} + frac{sin(nx)}{n^2}right)|_0^pi$$



Assuming that $sin(nx)$ is always $0$ and $cos(nx)$ is $(-1)^n$. Is that ok??



$$ = frac{1}{pi}cdotleft(frac{pi cdot (-1)^n}{n}right) = frac{picdot(-1)^n}{pi n} = frac{(-1)^n}{n}$$ voila, thats wrong again :-)



A solution would be great, but I also need to understand what was my mistake.



Thank you in advance!










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$endgroup$












  • $begingroup$
    This is an old post, but for anyone coming to this now: one issue I see is the integral for a0 at the beginning. It should be done from -pi to pi, not zero to pi.
    $endgroup$
    – Jake
    Dec 5 '18 at 5:05










  • $begingroup$
    @Jake This is obviously not an issue at all.
    $endgroup$
    – Did
    Dec 5 '18 at 7:14
















1












$begingroup$


I tried the following to create the Fourier-series of the function:



$$ f(x) = begin{cases} x & 0<x<pi \ 0 & pi < x < 2 pi end{cases}$$



This is what I tried:



$$a_0 = frac{1}{pi}intlimits_0^pi x dx = frac{1}{pi}cdotleft(frac{x^2}{2}|_0^piright) = frac{1}{pi}cdotfrac{pi^2}{2} = frac{pi^2}{2cdotpi} = frac{pi}{2}$$



Is that right?



$$a_n = frac{1}{pi}intlimits_0^pi x cdot cos(nx) dx = frac{1}{pi}cdotleft(frac{x cdot sin(nx)}{n} + frac{cos(nx)}{n^2}right)|_0^pi$$



Assuming that $sin(nx)$ is always $0$ and $cos(nx)$ is $(-1)^n$. Is that ok??



$$ = frac{1}{pi}cdotleft(frac{0}{n} + frac{(-1)^n}{n^2}right) = frac{1}{pi}cdotleft(frac{(-1)^n}{n^2}right) = frac{(-1)^n}{pi n^2}$$



That's already wrong I'm sure!



$$b_n = frac{1}{pi}intlimits_0^pi x sin(nx) dx = frac{1}{pi}cdotleft(frac{x cdot cos(nx)}{n} + frac{sin(nx)}{n^2}right)|_0^pi$$



Assuming that $sin(nx)$ is always $0$ and $cos(nx)$ is $(-1)^n$. Is that ok??



$$ = frac{1}{pi}cdotleft(frac{pi cdot (-1)^n}{n}right) = frac{picdot(-1)^n}{pi n} = frac{(-1)^n}{n}$$ voila, thats wrong again :-)



A solution would be great, but I also need to understand what was my mistake.



Thank you in advance!










share|cite|improve this question











$endgroup$












  • $begingroup$
    This is an old post, but for anyone coming to this now: one issue I see is the integral for a0 at the beginning. It should be done from -pi to pi, not zero to pi.
    $endgroup$
    – Jake
    Dec 5 '18 at 5:05










  • $begingroup$
    @Jake This is obviously not an issue at all.
    $endgroup$
    – Did
    Dec 5 '18 at 7:14














1












1








1





$begingroup$


I tried the following to create the Fourier-series of the function:



$$ f(x) = begin{cases} x & 0<x<pi \ 0 & pi < x < 2 pi end{cases}$$



This is what I tried:



$$a_0 = frac{1}{pi}intlimits_0^pi x dx = frac{1}{pi}cdotleft(frac{x^2}{2}|_0^piright) = frac{1}{pi}cdotfrac{pi^2}{2} = frac{pi^2}{2cdotpi} = frac{pi}{2}$$



Is that right?



$$a_n = frac{1}{pi}intlimits_0^pi x cdot cos(nx) dx = frac{1}{pi}cdotleft(frac{x cdot sin(nx)}{n} + frac{cos(nx)}{n^2}right)|_0^pi$$



Assuming that $sin(nx)$ is always $0$ and $cos(nx)$ is $(-1)^n$. Is that ok??



$$ = frac{1}{pi}cdotleft(frac{0}{n} + frac{(-1)^n}{n^2}right) = frac{1}{pi}cdotleft(frac{(-1)^n}{n^2}right) = frac{(-1)^n}{pi n^2}$$



That's already wrong I'm sure!



$$b_n = frac{1}{pi}intlimits_0^pi x sin(nx) dx = frac{1}{pi}cdotleft(frac{x cdot cos(nx)}{n} + frac{sin(nx)}{n^2}right)|_0^pi$$



Assuming that $sin(nx)$ is always $0$ and $cos(nx)$ is $(-1)^n$. Is that ok??



$$ = frac{1}{pi}cdotleft(frac{pi cdot (-1)^n}{n}right) = frac{picdot(-1)^n}{pi n} = frac{(-1)^n}{n}$$ voila, thats wrong again :-)



A solution would be great, but I also need to understand what was my mistake.



Thank you in advance!










share|cite|improve this question











$endgroup$




I tried the following to create the Fourier-series of the function:



$$ f(x) = begin{cases} x & 0<x<pi \ 0 & pi < x < 2 pi end{cases}$$



This is what I tried:



$$a_0 = frac{1}{pi}intlimits_0^pi x dx = frac{1}{pi}cdotleft(frac{x^2}{2}|_0^piright) = frac{1}{pi}cdotfrac{pi^2}{2} = frac{pi^2}{2cdotpi} = frac{pi}{2}$$



Is that right?



$$a_n = frac{1}{pi}intlimits_0^pi x cdot cos(nx) dx = frac{1}{pi}cdotleft(frac{x cdot sin(nx)}{n} + frac{cos(nx)}{n^2}right)|_0^pi$$



Assuming that $sin(nx)$ is always $0$ and $cos(nx)$ is $(-1)^n$. Is that ok??



$$ = frac{1}{pi}cdotleft(frac{0}{n} + frac{(-1)^n}{n^2}right) = frac{1}{pi}cdotleft(frac{(-1)^n}{n^2}right) = frac{(-1)^n}{pi n^2}$$



That's already wrong I'm sure!



$$b_n = frac{1}{pi}intlimits_0^pi x sin(nx) dx = frac{1}{pi}cdotleft(frac{x cdot cos(nx)}{n} + frac{sin(nx)}{n^2}right)|_0^pi$$



Assuming that $sin(nx)$ is always $0$ and $cos(nx)$ is $(-1)^n$. Is that ok??



$$ = frac{1}{pi}cdotleft(frac{pi cdot (-1)^n}{n}right) = frac{picdot(-1)^n}{pi n} = frac{(-1)^n}{n}$$ voila, thats wrong again :-)



A solution would be great, but I also need to understand what was my mistake.



Thank you in advance!







fourier-series






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share|cite|improve this question













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share|cite|improve this question








edited Jun 17 '13 at 10:21









Yoni Rozenshein

5,3201634




5,3201634










asked Jun 17 '13 at 10:08









Kevin MuhlerKevin Muhler

1015




1015












  • $begingroup$
    This is an old post, but for anyone coming to this now: one issue I see is the integral for a0 at the beginning. It should be done from -pi to pi, not zero to pi.
    $endgroup$
    – Jake
    Dec 5 '18 at 5:05










  • $begingroup$
    @Jake This is obviously not an issue at all.
    $endgroup$
    – Did
    Dec 5 '18 at 7:14


















  • $begingroup$
    This is an old post, but for anyone coming to this now: one issue I see is the integral for a0 at the beginning. It should be done from -pi to pi, not zero to pi.
    $endgroup$
    – Jake
    Dec 5 '18 at 5:05










  • $begingroup$
    @Jake This is obviously not an issue at all.
    $endgroup$
    – Did
    Dec 5 '18 at 7:14
















$begingroup$
This is an old post, but for anyone coming to this now: one issue I see is the integral for a0 at the beginning. It should be done from -pi to pi, not zero to pi.
$endgroup$
– Jake
Dec 5 '18 at 5:05




$begingroup$
This is an old post, but for anyone coming to this now: one issue I see is the integral for a0 at the beginning. It should be done from -pi to pi, not zero to pi.
$endgroup$
– Jake
Dec 5 '18 at 5:05












$begingroup$
@Jake This is obviously not an issue at all.
$endgroup$
– Did
Dec 5 '18 at 7:14




$begingroup$
@Jake This is obviously not an issue at all.
$endgroup$
– Did
Dec 5 '18 at 7:14










1 Answer
1






active

oldest

votes


















2












$begingroup$

Plugging in $x=0$ and $x=pi$ in $sin (nx)$ indeed gives you $0$.



Plugging in $x=pi$ in $cos (nx)$ indeed gives you $(-1)^n$.



But don't forget that plugging in $x=0$ in $cos (nx)$ gives you $1$.



Also, in the integral for $b_n$, there seems to be a minus sign missing after the integration by parts. Recheck your calculation.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    But don't forget that plugging in x=0 in cos(nx) gives you 1. --> Yes, that was an Issue in the an calculation. Should I correct this finding in the initial question or what would be the proper way on Stackexchange?
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 12:27












  • $begingroup$
    But I don't get where the minus in bn might be missing?
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 12:28










  • $begingroup$
    What is the anti-derivative of $sin x$?
    $endgroup$
    – Yoni Rozenshein
    Jun 17 '13 at 13:13










  • $begingroup$
    Since you asked about a mistake in your calculation, it is better to leave the mistake as-is in the question, so future readers will know what the question and answer were about.
    $endgroup$
    – Yoni Rozenshein
    Jun 17 '13 at 13:14










  • $begingroup$
    Uuups, right, the anti-derivative is: -(x * cos(nx) / n)) + (sin(nx) / n²)
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 13:38













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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









2












$begingroup$

Plugging in $x=0$ and $x=pi$ in $sin (nx)$ indeed gives you $0$.



Plugging in $x=pi$ in $cos (nx)$ indeed gives you $(-1)^n$.



But don't forget that plugging in $x=0$ in $cos (nx)$ gives you $1$.



Also, in the integral for $b_n$, there seems to be a minus sign missing after the integration by parts. Recheck your calculation.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    But don't forget that plugging in x=0 in cos(nx) gives you 1. --> Yes, that was an Issue in the an calculation. Should I correct this finding in the initial question or what would be the proper way on Stackexchange?
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 12:27












  • $begingroup$
    But I don't get where the minus in bn might be missing?
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 12:28










  • $begingroup$
    What is the anti-derivative of $sin x$?
    $endgroup$
    – Yoni Rozenshein
    Jun 17 '13 at 13:13










  • $begingroup$
    Since you asked about a mistake in your calculation, it is better to leave the mistake as-is in the question, so future readers will know what the question and answer were about.
    $endgroup$
    – Yoni Rozenshein
    Jun 17 '13 at 13:14










  • $begingroup$
    Uuups, right, the anti-derivative is: -(x * cos(nx) / n)) + (sin(nx) / n²)
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 13:38


















2












$begingroup$

Plugging in $x=0$ and $x=pi$ in $sin (nx)$ indeed gives you $0$.



Plugging in $x=pi$ in $cos (nx)$ indeed gives you $(-1)^n$.



But don't forget that plugging in $x=0$ in $cos (nx)$ gives you $1$.



Also, in the integral for $b_n$, there seems to be a minus sign missing after the integration by parts. Recheck your calculation.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    But don't forget that plugging in x=0 in cos(nx) gives you 1. --> Yes, that was an Issue in the an calculation. Should I correct this finding in the initial question or what would be the proper way on Stackexchange?
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 12:27












  • $begingroup$
    But I don't get where the minus in bn might be missing?
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 12:28










  • $begingroup$
    What is the anti-derivative of $sin x$?
    $endgroup$
    – Yoni Rozenshein
    Jun 17 '13 at 13:13










  • $begingroup$
    Since you asked about a mistake in your calculation, it is better to leave the mistake as-is in the question, so future readers will know what the question and answer were about.
    $endgroup$
    – Yoni Rozenshein
    Jun 17 '13 at 13:14










  • $begingroup$
    Uuups, right, the anti-derivative is: -(x * cos(nx) / n)) + (sin(nx) / n²)
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 13:38
















2












2








2





$begingroup$

Plugging in $x=0$ and $x=pi$ in $sin (nx)$ indeed gives you $0$.



Plugging in $x=pi$ in $cos (nx)$ indeed gives you $(-1)^n$.



But don't forget that plugging in $x=0$ in $cos (nx)$ gives you $1$.



Also, in the integral for $b_n$, there seems to be a minus sign missing after the integration by parts. Recheck your calculation.






share|cite|improve this answer









$endgroup$



Plugging in $x=0$ and $x=pi$ in $sin (nx)$ indeed gives you $0$.



Plugging in $x=pi$ in $cos (nx)$ indeed gives you $(-1)^n$.



But don't forget that plugging in $x=0$ in $cos (nx)$ gives you $1$.



Also, in the integral for $b_n$, there seems to be a minus sign missing after the integration by parts. Recheck your calculation.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Jun 17 '13 at 10:24









Yoni RozensheinYoni Rozenshein

5,3201634




5,3201634












  • $begingroup$
    But don't forget that plugging in x=0 in cos(nx) gives you 1. --> Yes, that was an Issue in the an calculation. Should I correct this finding in the initial question or what would be the proper way on Stackexchange?
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 12:27












  • $begingroup$
    But I don't get where the minus in bn might be missing?
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 12:28










  • $begingroup$
    What is the anti-derivative of $sin x$?
    $endgroup$
    – Yoni Rozenshein
    Jun 17 '13 at 13:13










  • $begingroup$
    Since you asked about a mistake in your calculation, it is better to leave the mistake as-is in the question, so future readers will know what the question and answer were about.
    $endgroup$
    – Yoni Rozenshein
    Jun 17 '13 at 13:14










  • $begingroup$
    Uuups, right, the anti-derivative is: -(x * cos(nx) / n)) + (sin(nx) / n²)
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 13:38




















  • $begingroup$
    But don't forget that plugging in x=0 in cos(nx) gives you 1. --> Yes, that was an Issue in the an calculation. Should I correct this finding in the initial question or what would be the proper way on Stackexchange?
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 12:27












  • $begingroup$
    But I don't get where the minus in bn might be missing?
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 12:28










  • $begingroup$
    What is the anti-derivative of $sin x$?
    $endgroup$
    – Yoni Rozenshein
    Jun 17 '13 at 13:13










  • $begingroup$
    Since you asked about a mistake in your calculation, it is better to leave the mistake as-is in the question, so future readers will know what the question and answer were about.
    $endgroup$
    – Yoni Rozenshein
    Jun 17 '13 at 13:14










  • $begingroup$
    Uuups, right, the anti-derivative is: -(x * cos(nx) / n)) + (sin(nx) / n²)
    $endgroup$
    – Kevin Muhler
    Jun 17 '13 at 13:38


















$begingroup$
But don't forget that plugging in x=0 in cos(nx) gives you 1. --> Yes, that was an Issue in the an calculation. Should I correct this finding in the initial question or what would be the proper way on Stackexchange?
$endgroup$
– Kevin Muhler
Jun 17 '13 at 12:27






$begingroup$
But don't forget that plugging in x=0 in cos(nx) gives you 1. --> Yes, that was an Issue in the an calculation. Should I correct this finding in the initial question or what would be the proper way on Stackexchange?
$endgroup$
– Kevin Muhler
Jun 17 '13 at 12:27














$begingroup$
But I don't get where the minus in bn might be missing?
$endgroup$
– Kevin Muhler
Jun 17 '13 at 12:28




$begingroup$
But I don't get where the minus in bn might be missing?
$endgroup$
– Kevin Muhler
Jun 17 '13 at 12:28












$begingroup$
What is the anti-derivative of $sin x$?
$endgroup$
– Yoni Rozenshein
Jun 17 '13 at 13:13




$begingroup$
What is the anti-derivative of $sin x$?
$endgroup$
– Yoni Rozenshein
Jun 17 '13 at 13:13












$begingroup$
Since you asked about a mistake in your calculation, it is better to leave the mistake as-is in the question, so future readers will know what the question and answer were about.
$endgroup$
– Yoni Rozenshein
Jun 17 '13 at 13:14




$begingroup$
Since you asked about a mistake in your calculation, it is better to leave the mistake as-is in the question, so future readers will know what the question and answer were about.
$endgroup$
– Yoni Rozenshein
Jun 17 '13 at 13:14












$begingroup$
Uuups, right, the anti-derivative is: -(x * cos(nx) / n)) + (sin(nx) / n²)
$endgroup$
– Kevin Muhler
Jun 17 '13 at 13:38






$begingroup$
Uuups, right, the anti-derivative is: -(x * cos(nx) / n)) + (sin(nx) / n²)
$endgroup$
– Kevin Muhler
Jun 17 '13 at 13:38




















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