Many phenomena which can be modeled through partial differential equations (PDEs), can be modeled as well with integral equations (IEs). Choosing IE over a PDE can be advantageous in many instances. One reason is that a PDE in s dimensions would require to be discretized into a set of simultaneous equations with Ns unknowns (N being related to the discretization); whereas its corresponding IE would require a set of Ns−1 unknowns. For example Hunt and Isaacs (1981), describe a formulation with IE of ground-water flow, explaining that the original boundary-value problem required a set of simultaneous equations with N2 unknowns, while its corresponding IE discretizes to only N unknowns.