On the characterization of exponential polynomials

Abstract.

Exponential polynomials, i.e. solutions of linear homogeneous differential equations with constant coefficients, are characterized as continuous solutions of linear homogeneous difference equations with constant coefficients. Namely, denote by C the ${\Bbb C}$-linear space of all continuous functions on ${\Bbb R}$, and define for $\omega \in {\Bbb R}_+$ the shift operator $z_{\omega }:C\to C$ by $\eta \mapsto z_{\omega }\eta$ such that $z_{\omega }\eta (t)=\eta (t+{\omega })$ for all $t\in {\Bbb R}$. Every ideal ${\mathfrak a}$ of $R:={\Bbb C}[x,y]$ determines a subspace¶¶$C_{\frak a}=\{\eta \in C:f(z_{{\omega }_1},z_{{\omega }_2})\eta =0$ for all $f(x,y)\in {\frak a}\}\,.$¶¶For incommensurable ${\omega }_1,{\omega }_2\in {\Bbb R}_+$ and ideals ${\frak a}$ in R with a factor ring $R/{\frak a}$ , which is finite dimensional as a ${\Bbb C}$-linear space, the set $C_{\mathfrak a}$ is characterized. The main idea consists in applying the Kronecker approximation theorem to the analytic transition from the explicit representation of the solutions of recurrence sequences to that of the continuous solutions of difference equations. For an alternative proof, the Kronecker approximation theorem is combined with the Jordan decomposition theorem.