Polynomial estimates, exponential curves and Diophantine approximation

Abstract

Let $\alpha\in(0,1)\setminus{\Bbb Q}$ and $K=\{(e^z,e^{\alpha z}):\,|z|\leq1\}\subset{\Bbb C}^2$. If $P$ is a polynomial of degree $n$ in ${\Bbb C}^2$, normalized by $\|P\|_K=1$, we obtain sharp estimates for $\|P\|_{\Delta^2}$ in terms of $n$, where $\Delta^2$ is the closed unit bidisk. For most $\alpha$, we show that $\sup_P\|P\|_{\Delta^2}\leq\exp(Cn^2\log n)$. However, for $\alpha$ in a subset ${\mathcal S}$ of the Liouville numbers, $\sup_P\|P\|_{\Delta^2}$ has bigger order of growth. We give a precise characterization of the set ${\mathcal S}$ and study its properties.