Lipshitzian Vector Fields, Upper Gradients And Distributional Derivatives

We prove that given a locally integrable function $f$ on an open set of an Euclidean space the distributional derivative $Xf$ with respect to a locally Lipshitzian vector field $X$ is locally integrable if, and only if, the function $f$ admits a locally integrable upper gradient along the vector field $X$; in this case $Xf$ coincides with the Lie derivative $L_{X}f$ and $\mathrm{\mid }Xf\mathrm{\mid }$ is the least upper gradient of the function $f$. Applications to systems of locally Lipshitzian vector fields are given.