Algebraic dynamics of skew-linear self-maps

Abstract:

Let $X$ be a variety defined over an algebraically closed field $k$ of characteristic $0$, let $N\in \mathbb {N}$, let $g:X\dashrightarrow X$ be a dominant rational self-map, and let $A:\mathbb {A}^N\longrightarrow \mathbb {A}^N$ be a linear transformation defined over $k(X)$, i.e., for a Zariski open dense subset $U\subset X$, we have that for $x\in U(k)$, the specialization $A(x)$ is an $N$-by-$N$ matrix with entries in $k$. We let $f:X\times \mathbb {A}^N\dashrightarrow X\times \mathbb {A}^N$ be the rational endomorphism given by $(x,y)\mapsto (g(x), A(x)y)$. We prove that if the determinant of $A$ is nonzero and if there exists $x\in X(k)$ such that its orbit $\mathcal {O}_g(x)$ is Zariski dense in $X$, then either there exists a point $z\in (X\times \mathbb {A}^N)(k)$ such that its orbit $\mathcal {O}_f(z)$ is Zariski dense in $X\times \mathbb {A}^N$ or there exists a nonconstant rational function $\psi \in k(X\times \mathbb {A}^N)$ such that $\psi \circ f=\psi$. Our result provides additional evidence to a conjecture of Medvedev and Scanlon.