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Part of ALG-02 — Complex Numbers

Cube Roots of Unity -- Complete Reference

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Let w = e^(2pii/3) = (-1 + i*sqrt(3))/2 (a primitive cube root of unity).

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

Useful derived identities:

  • (1 - w)(1 - w^2) = 3
  • (1 + w)(1 + w^2) = 1 -- since 1+w = -w^2 and 1+w^2 = -w, product = w^3 = 1
  • (1 - w + w^2)(1 + w - w^2) = 4 -- using 1+w+w^2=0
  • a + bw + cw^2 = 0 with a,b,c real implies a = b = c (if a,b,c are distinct, then they must satisfy specific conditions)
  • a^3 + b^3 = (a+b)(a+bw)(a+bw^2)
  • a^3 + b^3 + c^3 - 3abc = (a+b+c)(a+bw+cw^2)(a+bw^2+cw)

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