Pavel Solín Tomás Vejchodský Roberto Araiza Discrete conservation of nonnegativity for elliptic problems solved by the hp-FEM. 205-210 2007 76 Mathematics and Computers in Simulation 1-3 http://dx.doi.org/10.1016/j.matcom.2007.01.015 db/journals/mcs/mcs76.html#SolinVA07