Solution of the Dirichlet problem for the equation $a\Delta u+b\cdot \nabla u=0$ by the Monte Carlo method

In this paper we study the Dirichlet problem corresponding to an open bounded set $D\subset \mathbb{R}^{d}$ and the operator \begin{equation*} A=\sum_{i=1}^{d}a\frac{\partial ^{2}}{\partial x_{i}^{2}} +\sum_{i=1}^{d}b_{i}\frac{\partial }{\partial x_{i}}, \end{equation*} where $a>0$ and $b\in \mathbb{R}^{d}$. We define a mean value property and prove that a function $u$ has such property in $D$ if and only if $Au=0$ in $D$. Using this characterization, and a drifted Brownian motion, we define a family of random variables that converges almost surely and the limit is used to give an explicit representation for the solutions to the Dirichlet problem, this immediately implies the uniqueness. On the other hand, the existence of the solution is proved imposing a regular condition on the boundary of $D$.