# Taylor expand (1+z)^n where |z | < 1 and n is any complex #

Same as title.

## Homework Equations

Taylor expansion.

## The Attempt at a Solution

Okay - what?! I don't even know where to begin. I taylor expanded the function and pretended like n was just some number and that doesn't help. I've never learned this. How? Can you point me in some direction?

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FactChecker
Gold Member
I taylor expanded the function and pretended like n was just some number and that doesn't help.
Why not? You should show the work you did.

• Vitani11
Sorry if this is difficult to see (and sideways)

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FactChecker
Gold Member
I assume that this is a problem from a complex analysis class.
I don't know what you used as the definition of a Taylor series (should have been shown in Relevant equations) or if you know how to take the derivatives of that analytic function.
If you take the derivatives of an analytic function and use the definition of the Taylor series, it should be valid within it's radius of convergence.

• Vitani11
Ray Vickson
Homework Helper
Dearly Missed
Sorry if this is difficult to see (and sideways)
If you are serious about wanting help, you need to respect the helpers by typing out your solution.

• Vitani11 and FactChecker
Okay. Sorry.

1+nz/1!+(n(n-1)z2)/2!+(n(n-1)(n-2)z3)/3!+(n(n-1)(n-2)(n-3)z4)/4! is the expansion I get. I see the expansion for ez in there. What do I do about the complex parts (n?)

the complex parts of the expansion are n+n(n-1)+n(n-1)(n-2)+n(n-1)(n-2)(n-3) but they are all attached to a real coefficient Z so I can't split them. Do I need to generalize this pattern? because that is what my intuition tells me.

FactChecker
Gold Member
What bothers you about the complex parts? Are you in a complex analysis class?

• Vitani11
Mathematical physics - nothing bothers me about them I just need to know how to deal with them in an expansion.

FactChecker
Gold Member
When dealing with a function of complex variables, you should expect the Taylor series expansion to have complex coefficients. n-1 is a perfectly fine complex number if n is complex. But you should change the notation because everyone will assume that n is a natural number.

• Vitani11
Yes I know. So I expanded the function, as the question asked, but the next question then is show ln(1+z) = z-z2/2+z3/3-z4/4 using the results from the expansion I just did (I am not aloud to Taylor expand). I can't see a relationship between the expansion I just did and that question because the expansion I just did involves complex numbers whereas the next question has none in there. This is what I mean by I need to find a way to deal with the complex part of the expansion. I'm just trying to find relationships.

FactChecker
Gold Member
The best environment for Taylor series expansions are in complex numbers. Don't try to separate out the real and imaginary parts unless there is some unusual reason to.
n doesn't have to be complex, it just can be complex. You should be able to think of a value of n that is directly relates (1+z)n and ln(1+z).

I will not say more than that on a homework problem.

• Vitani11
Ray Vickson
• 