## Operators commuting with translations, and systems of difference equations

### Volume 80 / 1999

#### Abstract

Let ${\mathcal B} ={f:ℝ → ℝ: f is bounded}$, and ${\mathcal M} ={f:ℝ → ℝ: f is Lebesgue measurable}$. We show that there is a linear operator $Φ :{\mathcal B} → {\mathcal M}$ such that Φ(f)=f a.e. for every $f ∈ {\mathcal B} ∩ {\mathcal M}$, and Φ commutes with all translations. On the other hand, if $Φ : {\mathcal B} → {\mathcal M}$ is a linear operator such that Φ(f)=f for every $f ∈ {\mathcal B} ∩ {\mathcal M}$, then the group $G_Φ$ ={ a ∈ ℝ:Φ commutes with the translation by a} is of measure zero and, assuming Martin's axiom, is of cardinality less than continuum. Let Φ be a linear operator from $ℂ^ℝ$ into the space of complex-valued measurable functions. We show that if Φ(f) is non-zero for every $f(x)=e^{cx}$, then $G_Φ$ must be discrete. If Φ(f) is non-zero for a single polynomial-exponential f, then $G_Φ$ is countable, moreover, the elements of $G_Φ$ are commensurable. We construct a projection from $ℂ^ℝ$ onto the polynomials that commutes with rational translations. All these results are closely connected with the solvability of certain systems of difference equations.