What is the proof for cos(2pi/n) + isin(2pi/n) being a primitive root of unity?

In summary, the homework statement is trying to find a primitive root of unity and it states that if we set z = cos(2pi/n) + isin(2pi/n) then z^k cannot equal 1. If we use de moivres theorem to make z^k = cos(2kpi/n) + isin(2kpi/n), then I'm not certain what fact to use to write it as. However, if we set k<n, then k/n < 1, so (k/n)2.pi <2.pi, isn't that enough? No because sin(pi) is also zero and cos(pi) is 1 giving us 1 which
  • #1
Driessen12
18
0

Homework Statement


I must show that cos(2pi/n) + isin(2pi/n) is a primitive root of unity


Homework Equations


a primitive root of unity is an nth root of unity that does not equal 1 when raised to the kth power for k less than n and great than or equal to 1


The Attempt at a Solution


If we set z = cos(2pi/n) + isin(2pi/n) then z^k cannot equal 1. we can use de moivres theorem to make z^k = cos(2kpi/n) + isin(2kpi/n) and then I'm not certain what fact to use next
 
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  • #2
how about writing it as
[tex] cos(\frac{2 \pi}{n}} ) + i sin(\frac{2 \pi}{n}} ) = e^{\frac{2 \pi}{n}} [/tex]
 
  • #3
so then e^(2kpi/n) would be what i get. From there all i would need to show is that 2kpi/n cannot be zero, correct?
 
  • #4
lanedance said:
how about writing it as
[tex] cos(\frac{2 \pi}{n}} ) + i sin(\frac{2 \pi}{n}} ) = e^{\frac{2 \pi}{n}} [/tex]

But that's not true. You're off by a factor of i in the exponential.
 
  • #5
right, but to prove that z^k is not equal to zero i would have e^(i2πk/n) and this cannot equal 1 if we restrict k to be greater than or equal to 1 and less than n. So n cannot be zero and k cannot be zero, so i2(pi)k/n cannot be zero, thus z^k cannot equal 1 and is therefore primitive. and k is not equal to n so we can never have e^(i2(pi)). but how would i prove that the exponent can never be pi
 
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  • #6
yeah cheers mis-typed the i

well its a given that k<n, so k/n < 1, so (k/n)2.pi <2.pi, isn't that enough
 
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  • #7
no because sin(pi) is also zero and cos(pi) is 1 giving us 1 which is exactly what I have to prove cannot happen. For example choose k to be 2 and n to be 4, then we have e^(i(pi)) which is 1. I am not sure how to prove that this cannot happen though. I need to show that it is impossible to have any multiple of pi as my argument. any ideas?
 
  • #8
Driessen12 said:
no because sin(pi) is also zero and cos(pi) is 1 giving us 1 which is exactly what I have to prove cannot happen. For example choose k to be 2 and n to be 4, then we have e^(i(pi)) which is 1. I am not sure how to prove that this cannot happen though. I need to show that it is impossible to have any multiple of pi as my argument. any ideas?

You're doing something wrong if you think that cos(pi)=1. cos(pi)=-1.
 
  • #9
I wasn't thinking of course, you're right. I have it all proved now
 

What are primitive roots of unity?

Primitive roots of unity are complex numbers that, when raised to certain powers, result in all the other complex roots of unity. They are also referred to as primitive nth roots of unity, where n is a positive integer.

How are primitive roots of unity related to the roots of unity?

All primitive roots of unity are also roots of unity, but not all roots of unity are primitive roots of unity. The primitive roots of unity form a subset of the roots of unity.

What is the significance of primitive roots of unity?

Primitive roots of unity have important applications in mathematics, particularly in number theory and algebra. They are also used in signal processing and cryptography.

How do you find primitive roots of unity?

The formula for finding primitive roots of unity is given by ωn = e2πi/n, where n is the order of the root. Alternatively, you can also use a table or calculator to find the primitive roots of unity for a given order.

Can there be multiple primitive roots of unity for the same order?

Yes, there can be multiple primitive roots of unity for the same order. For example, there are two primitive fifth roots of unity, ω5 and ω5^2, both of which have an order of 5.

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