Think and solve

Algebra Level 5

For distinct positive integers p , q p, q , how many complex numbers z z satisfy the equation

z p = z ˉ q ? z ^ p = \bar{z} ^ q ?

Note: z ˉ \bar{z} is the complex conjugate of z . z.

1 Infinite None of these choices p + q + 1 p+q+1 p + q p+q p q p-q

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1 solution

Rodrigo Paniza
Dec 22, 2015

We first note that z = 0 z = 0 is a solution for all positive p , q p,q .

Next, every non-zero complex number can be written uniquely as z = r e i θ z = r \cdot e^{i\theta} where r > 0 , 0 θ < 2 π r>0,0\leq \theta < 2\pi . We note that this implies that z a = r a ||z^a|| = r^a when written in this form since e i x = 1 , x ||e^{ix}||=1, \forall x . Also, note that z ˉ = r e i θ \bar{z} = r\cdot e^{-i\theta} .

Thus, if the given equation is correct, then we first have

z p = z ˉ q r p = r q ||z^p|| = ||\bar{z}^q|| \Rightarrow r^p=r^q .

For positive real r r , this only has solutions at r = 1 r=1 . Thus our original problem is reduced to

e i θ p = e i θ q e i θ ( p + q ) = 1 e^{i\theta p} = e^{-i\theta q} \Rightarrow e^{i\theta (p+q)} = 1 .

From here, we see that we must have

θ ( p + q ) = 2 π n \theta \cdot (p+q) = 2\pi n

for natural n n , and restricting to the interval 0 θ < 2 π 0\leq \theta < 2\pi we have our entire set of solutions:

z = 0 , e 2 π i / ( p + q ) , e 2 2 π i / ( p + q ) , , e ( p + q 1 ) 2 π i / ( p + q ) \boxed{z=0,e^{2\pi i/(p+q)}, e^{2 \cdot 2\pi i/(p+q)}, \dots, e^{(p+q-1)\cdot 2\pi i/(p+q)} }

You're missing the solution z = e 0 2 π i / ( p + q ) = 1 z = e^{0\cdot 2\pi i/(p+q)} = 1 .

Ivan Koswara - 5 years, 5 months ago

I did the same thing but did not restrict myself to ( 0 , 2 π ) \left(0,2\pi\right)

A Former Brilliant Member - 5 years, 5 months ago

Why should we restrict in 0 2π

Akash aggrawal - 4 years, 8 months ago

put p=2 q=1 and calculate no. of functions and generalize it.easy

Ishan Dixit - 4 years, 2 months ago

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