Zhaoyin Xiang Chunlai Mu Lili Du A Liouville-type theorem for a system of parabolic inequalities and its applications. 976-981 2007 20 Appl. Math. Lett. 9 http://dx.doi.org/10.1016/j.aml.2006.10.002 db/journals/appml/appml20.html#XiangMD07