The authors consider operators of the form $L=\sum_{i=1}^{n}X_{i}^{2}+X_{0}$ in a bounded domain of $\mathbb{R}^{p}$ where $X_{0},X_{1},\ldots,X_{n}$ are nonsmooth Hormander's vector fields of step $r$ such that the highest order commutators are only Holder continuous. Applying Levi's parametrix method the authors construct a local fundamental solution $\gamma$ for $L$ and provide growth estimates for $\gamma$ and its first derivatives with respect to the vector fields. Requiring the existence of one more derivative of the coefficients the authors prove that $\gamma$ also possesses second derivatives, and they deduce the local solvability of $L$, constructing, by means of $\gamma$, a solution to $Lu=f$ with Holder continuous $f$. The authors also prove $C_{X,loc}^{2,\alpha}$ estimates on this solution.
Introduction
Some known results about nonsmooth Hormander's vector fields
Geometric estimates
The parametrix method
Further regularity of the fundamental solution and local solvability of $L$
Appendix. Examples of nonsmooth Hormander's operators satisfying assumptions
A or B
Bibliography.
Marco Bramanti, Politecnico di Milano, Italy.
Luca Brandolini, Universita di Bergamo, Dalmine, Italy.
Maria Manfredini, Universita di Bologna, Italy.
Marco Pedroni, Universita di Bergamo, Dalmine, Italy.
