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

1. Jan 20, 2017

### Vitani11

1. The problem statement, all variables and given/known data
Same as title.

2. Relevant equations
Taylor expansion.

3. 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?

2. Jan 21, 2017

### FactChecker

Why not? You should show the work you did.

3. Jan 21, 2017

### Vitani11

Sorry if this is difficult to see (and sideways)

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4. Jan 21, 2017

### FactChecker

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.

5. Jan 21, 2017

### Ray Vickson

If you are serious about wanting help, you need to respect the helpers by typing out your solution.

6. Jan 22, 2017

### Vitani11

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?)

7. Jan 22, 2017

### Vitani11

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.

8. Jan 22, 2017

### FactChecker

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

9. Jan 22, 2017

### Vitani11

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

10. Jan 22, 2017

### FactChecker

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.

11. Jan 22, 2017

### 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.

12. Jan 22, 2017

### FactChecker

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.

13. Jan 22, 2017

### Ray Vickson

The algebra is exactly the same as it would be if $n$ were a real number. The fact are that standard algebra holds for complex quantities, so that if some or all of the quantities are complex the basic laws $a+b = b+a$, $ab = ba$, $a(b+c) = ab + ac$, $a+0=a$, $a \times 1 = a$, $1/(1/a) = a$, etc., etc. all hold.

14. Jan 23, 2017

### Vitani11

Got it. Thanks amigos