@article{IM_1982__69_142948,
     author = {E.M. Stein},
     title = {An {Example} on the {Heisenberg} {Group} {Related} to the {Levy} {Operator.}},
     journal = {Inventiones mathematicae},
     pages = {209--216},
     publisher = {mathdoc},
     volume = {69},
     year = {1982},
     zbl = {0515.58032},
     url = {http://geodesic.mathdoc.fr/item/IM_1982__69_142948/}
}