z^9 -1 = (z^3 - 1)(z^6 + z^3 + 1)
.: the roots of z^6 + z^3 + 1 = 0 are among the roots of z^9 -1 = 0.
The roots of z^9 -1 = 0 are 1cis(2npi/9) where n = 0, 1, 2, 3, ......, 8 or
n = 0, +/-1, +/-2, +/-3, +/-4.
.: the roots of z^6 + z^3 + 1 = 0 are 1cis(2npi/9) where n = 1, 2, 4, 5, 7, 8...