 Eisenstein norm

For an Eisenstein integer $z=a+b\omega$, with $\omega =(-1+i\sqrt{3})/2$, the norm is $N(z)=z\overline{z}=a^2-ab+b^2$. It is a positive integer for $z\ne 0$, since $N(z)=(a-b/2{)}^2+3b^2/4$, and is multiplicative by the multiplicativity of the complex modulus. An element is a unit exactly when its norm is one: one direction follows from multiplicativity and integrality, and the other from the inverse $z^{-1}=\overline{z}$.