Finding the Function Represented by a Power Series

Click For Summary

Homework Help Overview

The discussion revolves around determining the coefficients \( a_n \) in the context of a power series equation involving sums of the form \( \sum_{n=1}^{\infty}{na_{n}x^{n-1}} + 2\sum_{n=0}^{\infty}{a_{n}x^{n}} = 0 \) and identifying the function represented by the series \( \sum_{n=0}^{\infty}{a_{n}x^{n}} = 0 \).

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss combining series and deriving relationships between coefficients. There are attempts to express \( a_{n+1} \) in terms of \( a_n \) and to identify a differential equation related to the power series. Questions arise about the correctness of approaches and the next steps in solving for \( a_n \).

Discussion Status

Participants are exploring different methods to derive the coefficients, including using differential equations and direct relationships between terms. There is recognition of the need for initial conditions, such as \( a_0 \), to proceed with the solution. Multiple interpretations of the problem are being considered without reaching a consensus.

Contextual Notes

Participants note the importance of the initial term \( a_0 \) in determining subsequent coefficients and question the implications of the derived relationships. The discussion reflects a collaborative effort to clarify the setup and assumptions of the problem.

popo902
Messages
59
Reaction score
0

Homework Statement



Determine the an so that the equation
[itex]\sum_{n=1}^{\infty}{na_{n}x^{n-1}} + 2\sum_{n=0}^{\infty}{a_{n}x^{n}} = 0[/itex]


is satisfied. Try to identify the function represented by the series
[itex]\sum_{n=0}^{\infty}{a_{n}x^{n}} = 0[/itex]


Homework Equations





The Attempt at a Solution




what i have so far is

[itex]\sum_{n=0}^{\infty}x^{n}[{a_{n+1}(n+1) + 2a_{n}}]= 0[/itex]

i just combined the series.
then i solved for a,n

an = -1/2(an+1)(n+1)

so...is this right?
if it is, where do i go from here?
 
Physics news on Phys.org
I would write it as a_{n+1}=(-2/(n+1))*a_{n}. But sure, that's ok. To identify the function I'd notice that one of those sums looks like the derivative of the other sum. Try and write a differential equation for
[itex] f(x)=\sum_{n=0}^{\infty}{a_{n}x^{n}}[/itex]
 
Last edited:
Or alternatively, you can use the original given equation to find [tex]a_0[/tex], then derive a formula for [tex]a_n[/tex] from that. It sounds like that's what they want. The differential equation method mentioned by Dick, however, is much quicker if you are allowed that.
 
i noticed from the first equation that the left sum, was the deriv. of the right, ecxept that the 2 was in front
so technically it would look like this : y' + 2y = 0, when y = [itex]\sum_{n=0}^{\infty}{a_{n}x^{n}} = 0[/itex]

so are you saying i should just solve the normal DE and i'll get the solution that resembles that summation...?


"I would write it as a_{n+1}=(-2/(n+1))*a_{n}."
i see now that this way was better since you need a0 to find the rest
...but then i thought i was solving for an?
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
2K
Replies
8
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 14 ·
Replies
14
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K