Part of ALG-02 — Complex Numbers

Cube Roots of Unity -- Complete Reference

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Let w = e^2pii3\frac{2*pi*i}{3} = (-1 + i*sqrt(3))/2 (a primitive cube root of unity).

IdentityValue
w3w^31
1 + w + w2w^20
w2w^2w-bar = (-1 - i*sqrt(3))/2
w * w2w^2w3w^3 = 1
|w|1
arg(w)2*pi/3
w^(-1)w2w^2

Useful derived identities:

  • (1 - w)(1 - w2w^2) = 3
  • (1 + w)(1 + w2w^2) = 1 -- since 1+w = -w2w^2 and 1+w2w^2 = -w, product = w3w^3 = 1
  • (1 - w + w2w^2)(1 + w - w2w^2) = 4 -- using 1+w+w2w^2=0
  • a + bw + cw2cw^2 = 0 with a,b,c real implies a = b = c (if a,b,c are distinct, then they must satisfy specific conditions)
  • a3a^3 + b3b^3 = (a+b)(a+bw)(a+bw2bw^2)
  • a3a^3 + b3b^3 + c3c^3 - 3abc = (a+b+c)(a+bw+cw2cw^2)(a+bw2+cwbw^{2+cw})

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