Fun with Cube root of unity!

Algebra Level 5

Find ( ω ω ) \Re (\omega^\omega ) , where ω \omega is the cube root of unity with positive imaginary part. Consider the priciple branch for the answer.

Notation: ( z ) \Re (z) denotes the real part of complex number z z .


This is a part of my set Aniket's Challenges in Mathematics


The answer is 0.0815.

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2 solutions

Chew-Seong Cheong
Feb 10, 2017

x = ( ω ω ) = ( exp ( 2 π 3 i e 2 π 3 i ) ) = ( exp ( 2 π 3 i ( cos 2 π 3 + i sin 2 π 3 ) ) ) = ( exp ( 2 π 3 i ( 1 2 + 3 2 i ) ) ) = ( exp ( π 3 i π 3 ) ) = ( e π 3 ( cos π 3 i sin π 3 ) ) = e π 3 cos π 3 = 1 2 e π 3 0.0815 \begin{aligned} x & = \Re \left( \omega^\omega \right) \\ & = \Re \left(\exp \left(\frac {2\pi}3 i\cdot e^{\frac {2\pi}3 i}\right) \right) \\ & = \Re \left(\exp \left(\frac {2\pi}3 i \left(\cos \frac {2 \pi}3 + i \sin \frac {2 \pi}3\right)\right) \right) \\ & = \Re \left(\exp \left(\frac {2\pi}3 i \left(-\frac 12 + \frac {\sqrt 3}2 i \right)\right) \right) \\ & = \Re \left(\exp \left(-\frac {\pi}3 i - \frac \pi{\sqrt 3} \right) \right) \\ & = \Re \left(e^{-\frac \pi{\sqrt 3}} \left(\cos \frac {\pi}3 - i \sin \frac {\pi}3 \right) \right) \\ & = e^{-\frac \pi{\sqrt 3}} \cos \frac {\pi}3 \\ & = \frac 12 e^{-\frac \pi{\sqrt 3}} \\ & \approx \boxed{0.0815} \end{aligned}

Exactly!! Well explained !

Aniket Sanghi - 4 years, 3 months ago

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i was getting a minus sign firstly (

hiroto kun - 4 years, 3 months ago
Prakhar Bindal
Feb 10, 2017

Ha ha nice one.

Firstly w = e^(i2pi/3)

then w in power is (-1+iroot3)/2

Then used euler's form of complex numbers

to get the complex number as (1-iroot3)/2[(e)^(pi/root3)]

calculating this is a pretty herculian task :P

Did the same.... (+1)!

Samarth Agarwal - 4 years, 3 months ago

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