On conjugacy of additive actions in the affine Cremona group

An additive action on an irreducible algebraic variety $X$ is an effective action ${\mathbb{G}}_a^n\times X\to X$ with an open orbit of the vector group ${\mathbb{G}}_a^n$. Any two additive actions on $X$ are conjugate by a birational automorphism of $X$. We prove that, if $X$ is the projective space, the conjugating element can be chosen in the affine Cremona group and it is given by so-called basic polynomials of the corresponding local algebra.