Solving a Linear ODE using a power series

In summary, the conversation is about finding the solution to a second-order differential equation using power series. The student is struggling with finding the value of C1 and putting the solution in the required summation form. They receive help in deriving the recurrence relation and finding the coefficients, but make a mistake in the power of the denominator. Eventually, they are able to correct the mistake and get the correct solution. The expert also mentions being able to help with a similar question in another thread.
  • #1
yecko
Gold Member
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Homework Statement


45F8F685-B056-4297-9F96-5679CA7A39EC.jpeg


Homework Equations


Power series
ODE

The Attempt at a Solution


1B4BB3F1-8717-4BB3-9B73-9668905861F9.jpeg
[/B]

Sorry for not typing all those things out from my phone..
How can I get C1?
And how can I put the solution in the required format? (I don't know how to put it in summation sign... and i cannot even solve out for r in the question...)
Thank you so much for any help!
 

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  • #2
yecko said:
cannot even solve out for r in the question
Small wonder: you leave out ##r## from the derivatives of ##y##. Except in ##C_n##, you want to replace ##n## by ##n+r##
 
  • #3
Thank you.
However even i used r+n, i still cannot make it into the required form... did i have something wrong with it? Or what should I do next to obtain the answer in the required format? Thank you.
7AD2046A-E29F-40D7-A423-DE4EDFF2A2D2.jpeg
 

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  • #4
You have a mistake in the equation for r. It should be r2+4r+4=0
 
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  • #5
Thanks for pointing this out...
5A68D7FB-3FFF-40CB-859B-1E6F9B9DCE44.jpeg

r=-2 which is now correct,
However, i cannot solve out the denominator of the summation part... how can I write it in form of summation?
Thank you very much!
 

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  • #6
Substitute r = -2 and simplify the denominator of an.
 
  • #7
I have already substituted r=-2, but i don't know the recurrance relation of that denominator... do you mind to give me some hints or relationship related?
Thank you
 
  • #8
Type in the recurrence relation of an, please. Your handwriting is blurred,
yecko said:
I have already substituted r=-2, but i don't know the recurrance relation of that denominator... do you mind to give me some hints or relationship related?
Thank you
You got the recurrence relation for an:
$$a_n=\frac{4 a_{n-1}}{(n+r)(n+r-1)+5(r+n)+4}$$
Substitute r=-2 and simplify. What do you get?
 
  • #9
Sorry for blur image... let me post a clearer image here:
B63C09BC-82B5-41CB-8B36-0376F400634F.jpeg

I have got my answer as 4^n/(n!), yet it is wrong as by the submission system... what's wrong with my answer? Thank you
 

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  • #10
8BC9FE35-EDFB-4F08-8F77-64432801FEB9.jpeg
 

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  • #11
yecko said:
Sorry for blur image...
View attachment 223115
I have got my answer as 4^n/(n!), yet it is wrong as by the submission system... what's wrong with my answer? Thank you
The recurrence relation for an is

##a_n=\frac {4 a_{n-1}}{n^2}##, with a0=1.
How did you get n! in the numerator?
 
  • #12
As I have to write the answer in terms of n only as a summation form (refer to the question image I have posted in #1), only writting it in recurrance relationship is not enough...
As the image I posted in #9 post explained how i got “n!”, when n tends larger, the relationship a(n) tends to be n! in denominator

4^n*(n!)/(n!)^2 =4^n/(n!)
 
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  • #13
I have already tried my best in obtaining the answer, do you mind to lead me what should I write in the blank as of the question image posted in post #10 please?
Thank you so much for your help!
 
  • #14
yecko said:
I have already tried my best in obtaining the answer, do you mind to lead me what should I write in the blank as of the question image posted in post #10 please?
Thank you so much for your help!
I do not understand what you mean on "summation form"
You need to find the coefficients cn in the power series solution.
You derived the recurrence relation $$c_n=\frac{4}{n^2}c_{n-1}$$, but you wrote wrong results for c1, c2, c3, c4...
c0=1
$$c_1=\frac{4}{1^2}c_{0}= \frac{4}{1^2}$$
$$c_2=\frac{4}{2^2}c_{1}=\frac{4^2}{2^2\cdot1^2}=\frac{4^2}{(2!)^2}$$
$$c_3=\frac{4}{3^2}c_{2}=??$$
and so on. So what is cn?
 
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1. How do you solve a linear ODE using a power series?

To solve a linear ODE using a power series, you first need to rewrite the ODE in terms of a power series. This involves substituting the power series into the ODE and equating coefficients of like powers. Then, you need to find the recurrence relation by solving for the coefficients. Finally, you can use the initial conditions to determine the values of the coefficients and thus find the solution to the ODE.

2. What is a power series?

A power series is a series of the form ∑n=0∞ an(x-x0)n, where an are the coefficients and x0 is the center of the series. It is an infinite series of terms with increasing powers of x. Power series are often used in calculus to approximate functions.

3. Why is it useful to solve ODEs using power series?

Solving ODEs using power series allows for the determination of an exact solution, rather than just an approximation. This is useful in situations where an exact solution is necessary, such as in physics and engineering problems. Additionally, power series solutions can be used to find solutions to ODEs that cannot be solved using other methods.

4. What are some applications of power series in solving ODEs?

Power series solutions to ODEs are used in a variety of fields, including physics, engineering, and economics. They can be used to model physical systems, such as in the study of fluid dynamics or electrical circuits. Power series solutions are also used in optimization problems, where the goal is to find the maximum or minimum of a function.

5. What are some challenges in solving ODEs using power series?

One of the main challenges in solving ODEs using power series is finding the recurrence relation for the coefficients. This often involves complex algebraic manipulations and can be difficult for higher-order ODEs. Additionally, power series solutions may not always converge, so it is important to check for convergence when using this method.

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