Second-order derivations of function spaces -- a characterization of second-order differential operators

Let $\mathrm{\Omega }\subset \mathbb{R}$ be a nonempty and open set, then for all $f,g,h\in {\mathcal{C}}^2(\mathrm{\Omega })$ we have \begin{multline*} \diff{2}{x}(f\cdot g\cdot h) -f\diff{2}{x}(g\cdot h)-g\diff{2}{x}(f\cdot h)-h\diff{2}{x}(f\cdot g) + f\cdot g\diff{2}{x}h+f\cdot h\diff{2}{x}g+g\cdot h\diff{2}{x}f=0 \end{multline*} The aim of this paper is to consider the corresponding operator equation $$D(f\cdot g\cdot h)-fD(g\cdot h)-gD(f\cdot h)-hD(f\cdot g)+f\cdot gD(h)+f\cdot hD(g)+g\cdot hD(f)=0$$ for operators $D\text{\hspace{0.17em}⁣}:{\mathcal{C}}^k(\mathrm{\Omega })\to \mathcal{C}(\mathrm{\Omega })$, where $k$ is a given nonnegative integer and the above identity is supposed to hold for all $f,g,h\in {\mathcal{C}}^k(\mathrm{\Omega })$. We show that besides the operators of first and second derivative, there are more solutions to this equation, and we characterize all solutions. Some special cases characterizing differential operators are also studied.