@article{MZ_1992__211_3_174454,
     author = {Shing-Tung Yau and Huai-Dong Cao},
     title = {Gradient estimates, {Harnack} inequalities and estimates for heat kernels of the sum of squares of vector fields.},
     journal = {Mathematische Zeitschrift},
     pages = {485--504},
     publisher = {mathdoc},
     volume = {211},
     number = {3},
     year = {1992},
     zbl = {0808.58037},
     url = {http://geodesic.mathdoc.fr/item/MZ_1992__211_3_174454/}
}