@article{MM2_1989__64_2_155409,
     author = {L. Boccardo and B. Dacorogna},
     title = {Monotonicity of certain differential operators in divergence form.},
     journal = {Manuscripta mathematica},
     pages = {253--260},
     publisher = {mathdoc},
     volume = {64},
     number = {2},
     year = {1989},
     zbl = {0718.47026},
     url = {http://geodesic.mathdoc.fr/item/MM2_1989__64_2_155409/}
}