Question about nth root of unity

[mesa]

Homework Statement

Determine the nth roots of unity by aid of the Argand diagram...

Homework Equations

Is the nth root of unity where we have a complex number to the 'nth' power equal to 1? For example,

$$(2+i)^n=1$$

The Attempt at a Solution

None yet, still trying to translate the question.

 

[SammyS]

Homework Statement

Determine the nth roots of unity by aid of the Argand diagram...

Homework Equations

Is the nth root of unity where we have a complex number to the 'nth' power equal to 1? For example,

$$(2+i)^n=1$$

The Attempt at a Solution

None yet, still trying to translate the question.

It's more like:

Solve zⁿ = 1 for z , where z is complex.

 

[slider142]

Yes. For example, -\frac{1}{2} + i\frac{\sqrt{3}}{2} is one of the 3rd roots of unity because \left(-\frac{1}{2} + i\frac{\sqrt{3}}{2}\right)^3 = 1.

 

[SteamKing]

Homework Statement

Determine the nth roots of unity by aid of the Argand diagram...

Homework Equations

Is the nth root of unity where we have a complex number to the 'nth' power equal to 1? For example,

$$(2+i)^n=1$$

The Attempt at a Solution

None yet, still trying to translate the question.

In math in English, 'unity' is a quaint way of saying 'one'.

 

[Mark44]

Homework Statement

Determine the nth roots of unity by aid of the Argand diagram...

Homework Equations

Is the nth root of unity where we have a complex number to the 'nth' power equal to 1? For example,

$$(2+i)^n=1$$

The only integer n for which this is true is n = 0.

2 + i is a complex number whose magnitude is √5. Multiplying it by itself is effectively rotating it by a certain amount, and increasing the magnitude. In polar form, this is easier to see.
2 + i = √5e^iθ, where θ = tan^-1(1/2).
(2 + i)ⁿ = [√5e^iθ]ⁿ = (√5)ⁿe^inθ

The Attempt at a Solution

None yet, still trying to translate the question.

 

[mesa]

Okay, this makes much more sense now, so we are looking for x and y values that will have a magnitude (modulus z, or r) of 1 and we can use a generating function for complex numbers like so:

$$\left( \frac{1}{k} \right)+i\left( \frac{\sqrt{k^2-1}}{k} \right)$$

or this,

$$\left( \frac{\sqrt{k^2-1}}{k} \right)+i\left( \frac{1}{k} \right)$$

I worked really hard with Latex on those big parentheses only to decide I prefer them without :)

 

[Mark44]

This looks really nice - I like the big parens - but I don't see how this is answers your original question. An nth root of unity is a complex number z, such that zⁿ = 1. If you write z = 1e^iθ, then you have zⁿ = 1ⁿe^niθ.

My comment followed your question about whether (2 + i)ⁿ could equal 1.

 

[SammyS]

How is k related to n ?

 

[ChrisVer]

the parenthesis are wrong... z^{n}≠|z|^{n}
in other words, writing z^{2} you mean zz not zz^{*}

So if you take the 2nd root for example (n=2) of the formula you provided:
(\frac{1}{k}+i \frac{\sqrt{k^{2}-1}}{k})(\frac{1}{k}+i \frac{\sqrt{k^{2}-1}}{k})≠1...

If you want to use the form for z=x+iy
then
(x+iy)^{n}=1
can be terrible...but not impossible to show...
Using the Binomial theorem:
(x+iy)= \sum_{k=0}^{n} (n k) x^{k} (iy)^{n-k}
http://www.dummies.com/how-to/content/how-to-expand-a-binomial-that-contains-complex-num.html
and proceed accordingly...(probably you'll have to end up with sins and cos, as a taylor expansions?)

Of course that's totally tedious... it's faster to follow the already proposed idea of writing z=r e^{i\theta} (r=1)
from the last you can also deduce that x^{2}+y^{2}=r^{2}=1
or better put that (x,y) belong on the unitary "circle" (in fact it's a circle for n going to infinity, otherwise it's a "circle" created by n-points). So here again you see the sin,cos solutions (which must be on the complex plane)...

 

[mesa]

This looks really nice - I like the big parens - but I don't see how this is answers your original question. An nth root of unity is a complex number z, such that zⁿ = 1. If you write z = 1e^iθ, then you have zⁿ = 1ⁿe^niθ.

My comment followed your question about whether (2 + i)ⁿ could equal 1.

Thanks! Maybe they aren't so bad then :)

So I got this from your suggestions,

$$1=\cos \left( \frac{2Pik}{n} \right) + i\sin \left( \frac{2Pik}{n} \right)$$

What a remarkable answer, and so versatile! I used algebra to write another solution but so far have only been able to get one answer for the cubic.

How is k related to n ?

It's not really, went off on a tangent :P

the parenthesis are wrong... z^{n}≠|z|^{n}
in other words, writing z^{2} you mean zz not zz^{*}

So if you take the 2nd root for example (n=2) of the formula you provided:
(\frac{1}{k}+i \frac{\sqrt{k^{2}-1}}{k})(\frac{1}{k}+i \frac{\sqrt{k^{2}-1}}{k})≠1...

If you want to use the form for z=x+iy
then
(x+iy)^{n}=1
can be terrible...but not impossible to show...
Using the Binomial theorem:
(x+iy)= \sum_{k=0}^{n} (n k) x^{k} (iy)^{n-k}
http://www.dummies.com/how-to/content/how-to-expand-a-binomial-that-contains-complex-num.html
and proceed accordingly...(probably you'll have to end up with sins and cos, as a taylor expansions?)

Of course that's totally tedious... it's faster to follow the already proposed idea of writing z=r e^{i\theta} (r=1)
from the last you can also deduce that x^{2}+y^{2}=r^{2}=1
or better put that (x,y) belong on the unitary "circle" (in fact it's a circle for n going to infinity, otherwise it's a "circle" created by n-points). So here again you see the sin,cos solutions (which must be on the complex plane)...

Yup, I messed up and was headed the wrong direction but have it now. I love binomial theorem, I wish they went over it in our engineering program, at least we have breaks for playing with such delightful things.