Carnot-Carathéodory metrics associated to degenerate elliptic operators in three dimensions

This note is a companion paper to arXiv:1608.01630 [math.CA]. Here we generalize some of the geometric results of arXiv:1608.01630 [math.CA] to the case of a $3\times 3$ matrix function $A(x)\approx \mathrm{d}\mathrm{i}\mathrm{a}\mathrm{g}\{1,f(x_1),g(x_1)\}$. More precisely, we make explicit calculations of the geodesics in the Carnot-Carathéodory space associated to $A$, and provide estimates on the Lebesgue measures of metric balls centered at the origin in that space.