Polymath 6.1 Key -

[ P(\mathbfx) = \sum_i=1^n \omega^x_i \quad \text(where $\omega$ is a primitive 3rd root of unity) ]

Let $x_1, x_2, \dots, x_n$ be variables in $0,1,2$ (or $\mathbbF_3$). Consider: polymath 6.1 key

But the actual breakthrough came from (e.g., $\mathbbF_3^n$). A specific “key polynomial” used in the density increment argument was: x_n$ be variables in $0