@article{BUMI_2001_8_4B_2_a8,
     author = {Chicco, M. and Lancia, M. R.},
     title = {Generalized maximum principle and evaluation of the first eigenvalue for {Heisenberg-type} operators with discontinuous coefficients},
     journal = {Bollettino della Unione matematica italiana},
     pages = {441--456},
     publisher = {mathdoc},
     volume = {Ser. 8, 4B},
     number = {2},
     year = {2001},
     zbl = {1164.35328},
     mrnumber = {106365},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/BUMI_2001_8_4B_2_a8/}
}