nth roots of unit proofs question

http://i57.tinypic.com/1678fv9.png

For 4i)

i get to

k is an integer

I just proved it like that, not sure if its right

ii)

for even n

but for odd n

for the sum, i got zero

for the product:

for even n i got

since you have (-1) half the time and 1 half the time

for odd n

since you have one more -1 than 1

please help with the last one!

Re: nth roots of unit proofs question

Quote:

Originally Posted by

**Applestrudle**

Here is a suggestion on notation that can simplify this answer. Use the exponentiation notation.

$e^z=\exp(z)=|z|(\cos(\theta)+i~\sin(\theta))$ where $\theta=\text{Arg}(z))$

This notation makes conjugate notation easy: $\overline{\exp(z)}=\exp(\overline{z} )$

For the first part of this question let $\theta=\dfrac{2\pi}{n}~\&~\rho=\exp(i\theta)$

Now it is easy to list the roots: $\rho^k:~k=0,1,\cdots, k-1$, and $\overline{~\rho~}=\exp(-i\theta)$.

Note that $\prod\limits_{k = 0}^{n - 1} {{\rho ^k}} = 1$.

For the second part the n nth roots of $i$: define $\rho=\exp\left(\dfrac{\pi i}{2}\right)~\&~\xi=\exp\left(\dfrac{2\pi i}{n}\right)$

Then those roots can be listed as: $\rho\cdot\xi^k,~k=0,\cdots,k-1$.

Re: nth roots of unit proofs question

Quote:

Originally Posted by

**Plato** Here is a suggestion on notation that can simplify this answer. Use the exponentiation notation.

$e^z=\exp(z)=|z|(\cos(\theta)+i~\sin(\theta))$ where $\theta=\text{Arg}(z))$

This notation makes conjugate notation easy: $\overline{\exp(z)}=\exp(\overline{z} )$

For the first part of this question let $\theta=\dfrac{2\pi}{n}~\&~\rho=\exp(i\theta)$

Now it is easy to list the roots: $\rho^k:~k=0,1,\cdots, k-1$, and $\overline{~\rho~}=\exp(-i\theta)$.

Note that $\prod\limits_{k = 0}^{n - 1} {{\rho ^k}} = 1$.

For the second part the n nth roots of $i$: define $\rho=\exp\left(\dfrac{\pi i}{2}\right)~\&~\xi=\exp\left(\dfrac{2\pi i}{n}\right)$

Then those roots can be listed as: $\rho\cdot\xi^k,~k=0,\cdots,k-1$.

for the sum z1 +z2 + z3+ z4+ .... i did the sum from k =1 to k=n of and i got i used geometric series

x + x^2 and x^3 + x^4 .... x^n = (1-x^n)/(1-x)

for the product z1.z2.z3.z4.z5 ... I used the geometric series in the exponential since

and i got the sum of the product as equal to

for the nth roots of i I got

for the product

so the product is e^(ipi/2n + sum of 2pik/n) for k from 1 to n

for the sum, using the geometric series, I got (2pi/n)(-n), giving -2pi ?

then the final answer for the product is

Re: nth roots of unit proofs question

Quote:

Originally Posted by

**Plato** Here is a suggestion on notation that can simplify this answer. Use the exponentiation notation.

$e^z=\exp(z)=|z|(\cos(\theta)+i~\sin(\theta))$ where $\theta=\text{Arg}(z))$

This notation makes conjugate notation easy: $\overline{\exp(z)}=\exp(\overline{z} )$

For the first part of this question let $\theta=\dfrac{2\pi}{n}~\&~\rho=\exp(i\theta)$

Now it is easy to list the roots: $\rho^k:~k=0,1,\cdots, k-1$, and $\overline{~\rho~}=\exp(-i\theta)$.

Note that $\prod\limits_{k = 0}^{n - 1} {{\rho ^k}} = 1$.

For the second part the n nth roots of $i$: define $\rho=\exp\left(\dfrac{\pi i}{2}\right)~\&~\xi=\exp\left(\dfrac{2\pi i}{n}\right)$

Then those roots can be listed as: $\rho\cdot\xi^k,~k=0,\cdots,k-1$.

for iv) i got the product P equals

P = w1.w2.w3.w4.w5.w6....wn (the 1,2,3...n are subs)

since -w* is also a solution, you can say wk = -wk* so they repeat themselves (is this logic correct?) and

P = (w1)^2 (w2)^2 (w3)^2....(w n/2)^2

in the exponent there is the sum from k =1 to k = (n/2) of 4k.pi

but i don't know how to simplify the sum, do i just leave it as a sum, is this even correct?