Iterated Cartier Flux and Non-Finite Generation of Tame Polynomial Automorphism Groups over Finite Fields

Abstract: Let $k=\mathbb F_q$ be a finite field of characteristic $p$, and let [ U_n(k)={F\in\operatorname{TA}n(k):F(0)=0,\ JF(0)=I_n}. ] We prove that every special tame polynomial automorphism is repeatedly Cartier-admissible for the de Rham flux class associated with [ λ=x_1\,dx_2\wedge\cdots\wedge dx_n. ] Lowest-weight projection of the iterated Cartier descents gives additive characters [ χ{r,j}:U_n(k)\longrightarrow(k,+), \qquad r\ge0,\quad 1\le j\le n, ] and for every fixed group element all but finitely many of these characters vanish. Put [ S_{n,p}=\begin{cases} \mathbf N_{\ge1},&(n,p)=(2,2),\ \mathbf N_0,&\text{otherwise}. \end{cases} ] The joint character map is surjective onto [ \bigoplus_{s\in S_{n,p}}(k^n,+), ] and every fixed-coordinate submap onto [ \bigoplus_{s\in S_{n,p}}(k,+) ] admits an explicit group-theoretic section. Consequently, [ \dim_{\mathbb F_p} \frac{U_n(k)}{[U_n(k),U_n(k)]\,U_n(k)^{[p]}} =\aleph_0. ] The character $χ<em>{r,j}$ factors through the jet of order [ N_r=(n-1)(p^{r+1}-1), ] and this order is optimal for every $r\in S</em>{n,p}$. Since $U_n(k)$ has finite index in $\operatorname{TA}_n(k)$, the tame polynomial automorphism group over a finite field is finitely generated exactly in dimension one. In particular, this proves the Maubach--Willems finite-generation conjecture for $n\ge3$.