Non-homogeneous ODE via Power Series

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Discussion Overview

The discussion revolves around solving a non-homogeneous ordinary differential equation (ODE) using power series. Participants explore the application of power series to the equation y'' + 3y' + 2y = sin x, with initial conditions y(0) = 0 and y'(0) = 1, and specifically focus on evaluating y(0.1).

Discussion Character

  • Homework-related
  • Exploratory
  • Technical explanation

Main Points Raised

  • One participant seeks guidance on solving the ODE using power series and expresses confusion after attempting to change the entire expression into power series.
  • Another participant emphasizes the need to understand the original poster's background in differential equations and power series to provide effective help.
  • Some participants note that the ODE can be solved analytically, but the original poster is focused on the power series method.
  • The original poster describes difficulties in working with the power series for the sine function compared to the series for y and its derivatives.
  • A participant requests to see the original poster's power series expansion to identify specific difficulties.
  • The original poster shares their power series expansions but later expresses uncertainty about factoring specific terms in the series.
  • Ultimately, the original poster resolves their confusion by realizing they can expand the series further, indicating a moment of clarity.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to solve the ODE using power series, as the discussion includes various perspectives and unresolved questions about the method.

Contextual Notes

The discussion reflects limitations in the original poster's understanding of power series and their application to the ODE, as well as the need for clarity on specific mathematical steps involved in the expansion.

teddy_boo
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Hey there! I'm new here and I just want to ask anyone willing how to solve this problem using power series:

y"+3y'+2y= sin x

y(0)=0
y'(0)=1

Evaluate y(0.1)

Thanks! :smile:
 
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It is impossible to explain something like that without knowing what you already understand about differential equations and power series.

It would be better if you what you understand about this problem and where you have trouble. What have you tried?

(I suppose it would be tacky of me to suggest that this looks like a homework problem. You wouldn't do that, would you?)
 
You are aware this can be solved analytically, yes? Just making sure.
 
Yes, I'm aware it could be solved analytically. My problem lies with the fact that after having changed the entire expression into power series, I'm totally blank on what to do next. The power series for the sine function is complicated relative to the one for y and its derivatives. I'm looking for some way to enable the sine function to be workable. Do you have any hints at least? Or am I looking at an inappropriate way to solve the ODE?
 
You say you have "changed the entire expression into power series". Please show what you have. The power series for sin x is relatively simple and I would like to see what exactly is giving you trouble. I suspect I know what your difficulty is but I don't want to go into a long-winded explanation if I am wrong.
 
Okay, so here's the equation:

∑_(i=2)^∞▒〖i(i-1) a_i x^(i-2)+3∑_(i=1)^∞▒〖ia_i x^(i-1)+2∑_(i=0)^∞▒〖a_i x^i=∑_(i=0)^∞▒(〖(-1)〗^i x^(2i+1))/(2i+1)!〗〗〗
 
Here's another way to put it upon expansion:

(2a_2+6a_3 x+12a_4 x^2+⋯)+(3a_1+6a_2 x+9a_3 x^2+⋯)+(2a_0+2a_1 x+2a_2 x^2+⋯)=x-x^3/3!+x^5/5!+⋯
 
After expansion, I don't know how to factor x^3 and x^5 terms.
 
Oh, everything's okay now, I was able to get the answer. Haha, I'm an idiot. My problem's easily remedied by just expanding the series further. Thanks anyway :smile:
 

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