“…The expression (3) is usually referred to as geometrical source term, that is motivated by the applications to the mathematical modeling for hydrogeology: in the Saint-Venant system for shallow waters, for instance, and the groundwater flows of an aquifer, the function Z(x) describes the bottom topography with respect to the spatial coordinates (see [5] and also [29]). Some models for nonlinear age-dependent population dynamics comprise source terms like (3) (refer to [9] for the original formulation of such problems), and various birth-death dynamical processes including the effects of competition for food are represented in [6] through equations for the probability density with that type of multiplicative noise. Furthermore, the influence of variable parameters as expressed via (3) characterizes many differential systems which are relevant to physical phenomena : relaxation inside chemical reactions, presence of external potentials in gas dynamics, material elasticity with memory, integro-differential models for granular flows in [11], simple optimization strategies for PDEs with non-conservative flux under hyperbolic constraints (see [15], for example), and besides basic quantitative models of multi-dimensional tomography for medical imaging to reconstruct the internal structure of solid objects from external measurements (refer to [7]).…”