Power Series Expansion Homework: Multiplication & n-k Addition Method

In summary, the conversation is about a problem involving multiplication with power series. The person is stuck at a certain part and the answer involves a double sum. The conversation also mentions the use of the Cauchy product and provides equations for the coefficients.
  • #1
tak13
8
0

Homework Statement



I am doing this multiplication with power series and I am just stuck at this one and other questions that similar to this one.
http://img5.imageshack.us/img5/9526/img1261r.jpg

Homework Equations





The Attempt at a Solution



It seems that I suppose to add n-k wherever I see a "n" but it doesn't seem right.
The highlighted part is the part where I stuck.
The answer for this problem is the one that I circled.
 
Last edited by a moderator:
Physics news on Phys.org
  • #2
There should be a double sum there, with a sum over k from k = 0 to n. Then you should be able to just solve what that sum would be.
 
  • #3
Ah yes, I know the part I highlighted should have the Sum of N parenthesis then the Sum of K but I don't know how they get to that answer.
 
  • #4
tak13 said:
...

The Attempt at a Solution



It seems that I'm supposed to add n-k wherever I see a "n" but it doesn't seem right.
The highlighted part is the part where I stuck.
The answer for this problem is the one that I circled.
What do you get if you multiply the first few terms of f(x) times the first g(x) ?

(fg)(x) = (1 + 2(x-2) + 3(x-2)2 + 4(x-2)3 + ... ) (1 + (x-2) + (x-2)2 + (x-2)3 + ...)

What is the "k" in your sum?
 
  • #5
You are using the Cauchy product:

[tex](\sum_{n=0}^\infty a_nx^n)(\sum_{n=0}^\infty b_nx^n)=(\sum_{n=0}^\infty c_nx^n)[/tex]

where

[tex]c_n = \sum_{k=0}^n a_kb_{n-k}[/tex]

In your case ak = k+1 and bk = 1. Figure out what you get for cn.
 

1. What is a power series expansion?

A power series expansion is a mathematical representation of a function as an infinite sum of powers of a variable. It is a useful tool for approximating functions and solving differential equations.

2. How is a power series expansion written?

A power series expansion is typically written in the form of Σ an(xn), where an represents the coefficients and x represents the variable. The series continues infinitely, with each term increasing in power.

3. What is the purpose of using a power series expansion?

The purpose of using a power series expansion is to approximate a function that may be difficult to evaluate directly. It can also be used to find solutions to differential equations and to study the behavior of a function as the variable approaches certain values.

4. How is a power series expansion different from a Taylor series?

A Taylor series is a type of power series expansion, but it is specifically centered around a single point and can be used to represent a function in the form of a polynomial. A general power series expansion does not have to be centered around a specific point and may not necessarily represent a polynomial.

5. What are the applications of power series expansion in science?

Power series expansion has various applications in science, including in physics, chemistry, and engineering. It is commonly used to approximate physical phenomena, such as the behavior of particles in a magnetic field or the motion of a pendulum. It is also used to solve differential equations in many scientific fields.

Similar threads

  • Calculus and Beyond Homework Help
Replies
7
Views
698
  • Calculus and Beyond Homework Help
Replies
6
Views
1K
  • Calculus and Beyond Homework Help
Replies
29
Views
1K
  • Calculus and Beyond Homework Help
Replies
4
Views
1K
  • Calculus and Beyond Homework Help
Replies
5
Views
879
  • Calculus and Beyond Homework Help
Replies
9
Views
3K
  • Calculus and Beyond Homework Help
Replies
2
Views
3K
  • Calculus and Beyond Homework Help
Replies
5
Views
2K
  • Calculus and Beyond Homework Help
Replies
5
Views
1K
  • Calculus and Beyond Homework Help
Replies
5
Views
1K
Back
Top