in analysing nonlinear phenomena many mathematical models give rise to problems for which only nonnegative solutions make sense. in the last few years this discipline has grown dramatically. this state-of-the-art volume offers the authors recent work, reflecting some of the major advances in the field as well as the diversity of the subject. audience: this volume will be of interest to graduate students and researchers in mathematical analysis and its applications, whose work involves ordinary differential equations, finite differences and integral equations.