@article{MAN_1996__304_4_165425,
     author = {Nicola Garofalo and Zhongwei Shen},
     title = {Absence of positive eigenvalues for a class of subelliptic operators.},
     journal = {Mathematische Annalen},
     pages = {701--716},
     publisher = {mathdoc},
     volume = {304},
     number = {4},
     year = {1996},
     zbl = {0847.35094},
     url = {http://geodesic.mathdoc.fr/item/MAN_1996__304_4_165425/}
}