Part of ALG-02 — Complex Numbers

Cube Roots of Unity Deep Dive

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Cube roots of unity: 1, w, w2w^2 where w = (-1+i*sqrt(3))/2 = e^2pii3\frac{2*pi*i}{3}.

Fundamental Properties:

  • w3w^3 = 1 and 1 + w + w2w^2 = 0
  • w2w^2 = w-bar (they are conjugates)
  • |w| = |w2w^2| = 1
  • w^(-1) = w2w^2

Simplification Substitutions:

  • 1 + w = -w2w^2 (most used substitution)
  • 1 + w2w^2 = -w
  • w + w2w^2 = -1

Useful Products:

  • (1-w)(1-w2w^2) = 3
  • (1+w)(1+w2w^2) = 1
  • (a+bw+cw2cw^2)(a+bw2+cwbw^{2+cw}) = a^{2+b}^{2+c}^{2-ab-bc-ca}

Factorizations:

  • x2+x+1x^{2+x+1} = (x-w)(x-w2w^2)
  • x31x^{3-1} = (x-1)(x-w)(x-w2w^2)
  • a^{3+b}^{3+c}^{3-3abc} = (a+b+c)(a+bw+cw2cw^2)(a+bw2+cwbw^{2+cw})

Power Reduction: For any integer n, wnw^n = w^(n mod 3). This converts any power of w to one of {1, w, w2w^2}.

JEE Problem Strategy: When you see w in a problem:

  1. Immediately write 1+w+w2w^2 = 0
  2. Reduce all powers mod 3
  3. Replace 1+w with -w2w^2 (or similar)
  4. Simplify using w3w^3 = 1

These identities appear in determinants, binomial coefficients, and algebraic expression problems. Master the substitutions to solve in 30-60 seconds.

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