Power series (expansion series)

In summary, the expansion of the power series (logarithmic series), ln(1+x), has a condition for x to be between -1 and 1 because when x=-1, the series is undefined and when |x|>1, the series does not represent ln(x+1). This series is only valid for a narrow range of x values because it converges to ln(x+1) only within that range. The suggestion is to consider what it means for a function to converge and where the power series coincides with the function.
  • #1
delsoo
97
0

Homework Statement



hi, for the expansion of power series (logarithmitic series) , ln(1+X) , why the condition for x is between -1 and 1 which x can be 1 but x can't be -1 ?

Homework Equations





The Attempt at a Solution

 
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  • #3
Simon Bridge said:
What happens when x=-1 or |x|>1?
i.e. compare the series with what it is supposed to represent.

note: http://hyperphysics.phy-astr.gsu.edu/hbase/math/lnseries.html

IF x--1 , then y=ln(0) , which is undefined , if |x|>1, i would get x less than- 1 and x more than 1 ... sub x value (less than -1 ), i would get ln(-x) which is also undefined, sub x value(more than 1 ), i would get ln (infinity) what does it represent?
 
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  • #4
...what does it represent?
The power series you refer to is supposed to represent ln(x+1) ... so you have noticed that ln(x+1) is only defined for x>-1. This should tell you part of the answer to your question.

To understand the rest of your question, you have to refer to the series itself - what is the series?
What happens to the series when x=-1? Is this consistent with what happens to ln(x+1)?
What happens to the series when |x|>1? Is that consistent?

What I'm getting at is that the series is only valid for a narrow range of possible x values because those are the only values where the series, summed to infinity, is equal to ln(x+1).
 
  • #5
referring to my textbook, if x=1 , ln(1+X) converges, can you show me how it can converges please?
 
  • #6
...can you show me how it can converges please?
I can but I won't - that is perilously close to doing work for you, that you are best advised to do yourself.

You should be able to show yourself that it converges: consider what it means for a function to converge.
i.e. where does the power series coincide with the function?

You appear to be ignoring suggestions. If you do not follow suggestions nobody can help you.
You've run into this problem in other threads too.
 
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FAQ: Power series (expansion series)

What is a power series?

A power series is an infinite series of the form ∑n=0∞ an(x-c)n, where an are constants and c is a fixed number. It is a way of representing a function as a sum of terms with increasing powers of x.

What is the purpose of using a power series?

Power series are useful for approximating functions that are difficult to evaluate directly. By truncating the series at a finite number of terms, we can get an approximation of the original function that is often accurate enough for practical purposes.

How do you determine the convergence of a power series?

The convergence of a power series depends on the value of the variable x. A power series will converge for all values of x within a certain interval called the interval of convergence. This interval can be determined using the ratio test or the root test.

Can a power series represent any function?

No, not every function can be represented by a power series. The function must have a Taylor series expansion in order to be represented as a power series. Additionally, the interval of convergence for the power series may not include the entire domain of the function, so it may only be a valid approximation within a certain range of values.

How are power series used in real-world applications?

Power series are used in a variety of applications, including physics, engineering, and economics. They are particularly useful for approximating complex functions in order to make calculations more manageable. For example, they are used in electrical engineering to model alternating currents, and in economics to study inflation and interest rates.

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