“…Either because of lack of information about the true values for the parameters involved or in order to facilitate the numerical methods involved. For example, in some cases where the outward flux must be null, condition (2.10) is used instead of (2.8) [5,7,8,9,10,12,13,14,16,17,18,19,20,21,23]. In others, where there is a partial flux on the boundary, condition (2.16) is used instead of (2.13), [8,10,14,20].…”