What am I doing wrong in the Fourier expansion?

In summary, the conversation discusses solving the ODE {y}^{\prime\prime}(t) + \omega^2{y(t)} = {r(t)} given {r(t)} = cos\omega{t} for different values of \omega. The process involves finding the complementary solution using the form y = \exp{(mt)}, which results in the fundamental solutions \cos\omega{t} and \sin\omega{t}. For the particular solution, a Fourier series is used to expand cos\omega{t}, but there seems to be an issue with getting {a}_{0} and {a}_{n} to be non-zero. The final solution is given as {y} = {c}_{1
  • #1
Ne0
12
0
Ok we are given the ODE
[tex]
{y}^{\prime\prime}(t) + \omega^2{y(t)} = {r(t)}
[/tex]
[tex]
r(t) = cos\omega{t}
[/tex]
[tex] \omega = 0.5,0.8,1.1,1.5,5.0,10.0
[/tex]
I know you can use variation of paramaters to solve for it so I start by finding the complementary solution.
[tex]
{y}^{\prime\prime}(t) + \omega^2{y(t)} = 0
[/tex]
We know solutions are of the form
[tex]
y = \exp{(mt)}
[/tex]
so after taking derivatives and what not we get the fundamental solution
[tex]
\cos\omega{t}, \sin\omega{t}
[/tex]
Our complementary solution is
[tex]
{y}_{c}=Acos \omega{t} + Bsin \omega{t}
[/tex]
For the particular solution we set
[tex]
{y}^{\prime\prime}(t) + \omega^2{y(t)} = cos\omega{t}
[/tex]
We then use a Fourier series to expand
[tex]
cos\omega{t}
[/tex]
Then proceed to solve for it but the problem I'm having is that I'm getting the Fourier series to be zero which is strange. I know that there will be no
[tex]
{b}_{n}
[/tex]
term since cos is even but its still werid why I'm getting zero for
[tex]
{a}_{0}, {a}_{n}
[/tex]
Any help would be appreciated.
 
Last edited:
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  • #2
I swear this latex thing I can't figure it out.
 
  • #3
If you guys are stuck the answer in the book is:
[tex]
{y} = {c}_{1}\cos\omega{t} + {c}_{2}\sin\omega{t} + {A}(\omega)\cos\omega{t}
[/tex]

[tex]
{A}(\omega) = \frac{1}{\omega^2 - 1} {\leq} 0
[/tex]
if [tex] \omega^2 {\leq} 1 [/tex]

[tex]
{A}(\omega) = \frac{1}{\omega^2 - 1} {\geq} 0
[/tex]
if [tex] \omega^2 {\geq} 1 [/tex]

Since there was not only greater then and less then I had to use less then or equal and greater then or equal
 
Last edited:
  • #4
Can anyone help please with what I am doing wrong in the Fourier expansion?
 

FAQ: What am I doing wrong in the Fourier expansion?

What is an ODE with Fourier Series?

An ODE (ordinary differential equation) with Fourier series is a type of mathematical equation that combines the concepts of ordinary differential equations and Fourier series. It involves using Fourier series to solve for the solution of an ODE, which is a mathematical equation that relates the derivative of an unknown function to the function itself.

How is an ODE with Fourier Series solved?

First, the ODE is converted into a Fourier series representation by expressing the unknown function as a sum of sines and cosines. Then, the coefficients of the series are determined by using the properties of Fourier series. Finally, the coefficients are substituted back into the Fourier series representation to find the solution to the ODE.

What is the significance of using Fourier series in solving ODEs?

Using Fourier series to solve ODEs allows for the representation of a periodic function as an infinite sum of sines and cosines. This makes it possible to solve for solutions to ODEs that would otherwise be difficult or impossible to find using traditional methods.

What are some applications of ODEs with Fourier Series?

ODEs with Fourier series have numerous applications in various fields of science and engineering. They are commonly used in the study of heat transfer and diffusion, as well as in the analysis of electrical circuits and vibrations in mechanical systems.

Are there any limitations to using Fourier series to solve ODEs?

While Fourier series can be a powerful tool in solving ODEs, there are some limitations to its use. It is only applicable to linear ODEs and may not be able to find solutions for all types of boundary conditions. Additionally, the convergence of Fourier series may be slow for certain functions, making it less efficient for solving certain ODEs.

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