2. Calculations

To integrate a differential equation such as (4) , you need to know which terms can be considered constant (strictly speaking or as a first approximation), and which are likely to vary significantly.

This leads us to examine, on the one hand, the ideal case of incompressible cakes (which can, at best, be considered as an acceptable approach to certain filtrations), and on the other hand, the general case of compressible cakes.